SYNOPSISObservations made on the influence of the fixative on staining of thin tissue sections for electron microscopy permit the following statements:After acrolein fixation it is possible to demonstrate basic proteins of the chromatin by silver impregnation. The staining appears to be mediated by the product of the reaction between the fixative and the imidazole groups of histidine.In the absence of reduced osmium (fixation by formol or acrolein or preliminary oxidation of sections of osmium‐fixed tissue) conventional lead methods result in a selective staining of the ribonucleoproteins. Under the same conditions, a selective staining of the plasma membrane (or an associated substance) can be achieved by phosphotungstic acid.After formol or acrolein fixation, uranyl acetate (at pH 4.5 to 5) stains the desoxyribonucleoproteins more intensely than the ribonucleoproteins. The strongest differentiation between the nucleoproteins is found to occur after formol‐osmium fixat