Summary1. Enzymes and related substances may be localized in cells either,(a)by separation and analysis of cell parts, or(b)by application of light absorption methods (with or without staining) to living cells or tissue sections.2. Separation of cell parts has been achieved by microdissection, by centrifuga‐tion (sea‐urchin eggs), by sectioning (neurones or centrifugally stratified Amoebae) and by bulk centrifugation of tissue suspensions. Highly sensitive micromethods, based on titration, dilatometry, or manometry, have been evolved for measuring the enzyme content of small fragments of protoplasm.3. Precautions must be taken to minimize separation artefacts. In the bulk centrifugation techniques it is only possible to draw satisfactory conclusions when an enzyme remains associated with certain granules after repeated washing with physiological solutions. Even if an enzyme is constantly associated with, for instance, the mitochondria, it may occur only in certain of them, for they are not necessarily all identical. It is important(a)to distinguish between enzyme activity under the conditions existing in the cell and the maximum activity shown by an extract under optimal conditions,(b)to ensure that the total quantity of enzyme in the fractions separated equals that of the whole cell or tissue,(c)to distinguish between the concentration of, for instance, peptidase in any fraction and the proportion of the total cell peptidase present in the fraction.4. The investigations which have most nearly satisfied these requirements have led to the following conclusions:(a)In various nuclei, dipeptidase, alkaline and acid phosphatase, arginase, uricase and esterase have been detected, all in lower absolute amounts than in the cytoplasm and all except alkaline phosphatase, arginase and uricase in lower concentration. Chromosome‐like threads can be centrifuged from sperm and suspensions of somatic tissues; after removal of desoxyribonucleohistone, there remains a residual chromosome containing ribonucleic acid, alkaline phosphatase and a tryptophane‐containing protein as well as other components,(b)The major part of the cytochrome oxidase, of the succinic dehydrogenase and of several enzymes concerned in fatty acid oxidation and in the Krebs tricarboxylic acid cycle is present in the mitochondria in rat‐liver cells. Mitochondria contain neither dipeptidase nor catalase in sea‐urchin eggs and Amoebae, but in the latter they appear to be associated with amylase activity,(c)Microsomes (cytoplasmic particles 50–200m/j.in diameter) have been found to contain dipeptidase, catalase, phosphatases, ribonuclease, about half the esterase and half the ribonucleic acid in adult liver cells,(d)Much of the catalase, carboxylase and lactic acid dehydrogenase in yeast, of the glycolytic activity in rat liver and of the ribonucleic acid in embryonic tissues and tumours remains in the supernatant after centrifugation for 1–2 hr. at 18,000 g., whether because these substances are freely dissolved in the protoplasm, or because they are eluted from particles during separation is not clear.5. Techniques applied to whole cells or tissue sections involve absorption spectroscopy in visible or ultra‐violet light, fluorescence spectroscopy, colour reactions used as staining methods, digestion techniques which remove cell components of a certain composition, and the utilization of enzyme activity itself to produce, directly or indirectly, a coloured deposit at the enzyme site. Fixation by freezing and drying reduces fixation artefacts and uncertainties in many cases. Particularly useful are semi‐quantitative ultra‐violet absorption methods for nucleic acids and qualitative deposition methods for phosphatases. Methods for localizing nadi‐oxidase, peroxidase, dopa‐oxidase, amine oxidases, riboflavine, thiamine, zymohexase, glucuronidases, lipases and choline esterases have also been described, but some of these, in particular those for the oxidases, are unsatisfactory.6. Desoxyribonucleic acid hardly ever occurs outside the nucleus. During interphase it appears only in the so‐called heterochromatic regions of the chromosomes, but at mitosis the chromosomes become rich in desoxyribonucleic acid throughout their length, and in some cases show alternating bands rich and poor in this substance. After mitosis, the ribonucleoprotein‐rich nucleolus reappears and it has been suggested that, from the nucleolus and its associated chromatin, substances migrate through the nuclear membrane and induce the formation of ribonucleic acid in the cytoplasm. In some egg cells whole nucleolar buds undergo such a migration. After mitosis, the concentration of nucleic acid does, in fact, fall in the nucleus and rise in the cytoplasm, which is especially rich in ribonucleic acid in cells synthesizing protein during growth, regeneration and secretion.7. Information regarding the distribution of phosphatases in cells is considered under three headings:(a)cell‐border phosphatase in sites of rapid solute exchange: gut, kidney and other excretory organs, placental membranes and capillaries;(b)phosphatase in sites of normal and abnormal calcification;(c)phosphatase in sites of active nucleoprotein metabolism.8. Application of nadi‐oxidase, peroxidase and similar techniques suggests that in most, but not all, cases early determination in mosaic eggs is accompanied by regional differences in enzyme content, whereas regulative eggs show a more nearly uniform distribution until a later stage. The techniques involved are often, however, of uncertain validity and this important subject needs reinvestigation.9. Only brief reference is made to plant tissues. In the photosynthetic cells of green leaves, the enzymes concerned in carbon dioxide fixation appear to be situated in the protoplasm outside the chloroplasts, but it is the latter which are concerned with the photolysis of water liberating oxygen and the hydrogen which reduces the initial products of carbon dioxide fixation.10. General conclusions and possible future developments are briefly discussed with special reference to the use of radioactive isotopes and to the need for techniq