Summary1. If it were experimentally possible to examine muscle extracts made during defined phases of the contraction cycle, the relation of muscular work to changes in the composition of such extracts might then be elucidated. The changes of normal muscle proteins in relation to muscular function, could then be considered from a truly physiological point of view.2. Such a study is possible: the technique for making extracts(a)from resting and relaxed muscle, (6) from fatigued muscle, and(c)from contracted muscle, are described. Reasons are given for preferring certain extracting agents. Advantages of the electrophoretic method of analysis are discussed.3. Proteins extractable at μ≪ 0–15 can be assumed to be in solution in the muscle cell. Fatigue and contraction do not modify their qualitative or quantitative distribution in the extracts.4. Proteins appearing in an extract of an ionic strength>0–15 ≪ 0.5 must be assumed to be fixed in the cell as insoluble complexes; they can be released by strong salt solutions, which break down certain linkages. To this group belong actomyosin, β1bT2and γ myosins obtained from normal resting muscle.5. The qualitative and quantitative composition of extracts at μ 0–35‐0‐50 depends on the functional state of the muscle, which influences the protein linkages. Thus, during all kinds of contraction (maximal contraction) or contracture (contracture by monoiodoacetate, strychnine or rigor mortis), β1and β2myosins become inextractible, while a new protein, contractin, appears.6. These results are discussed. It appears that the contractile mechanism is characterized by the formation and dissociation of protein complexes whose constituents are numerous and mostly unidentified, practically nothing is known as to the nature of their linkage forces.7. This review deals with an early stage in the physiological study of muscle proteins: the problem is to determine which of the proteins really belong to the structures which take part in muscular contraction and relaxation–by the use of methods which test the strength of the bonds holding these proteins in position. The use of extracting agents of various ionic composition, which may act selectively on these bonds, may be expected in the future to yield important a