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Banga and Szent-GySrgyi (1) have found that two kinds of myosin can be obtained from rabbit muscle depending on the time of extraction of the muscle. Muscle extracted for 24 hours yields myosin which is very viscous (myosin B) and the viscosity drops on the addition of adenosine triphosphate (ATP), while muscle extracted for 20 minutes yields myosin which is less viscous and the viscosity does not change on addition of ATP (myosin A). Straub (2), studying the properties of the two forms of myosin, obtained a highly viscous protein extract from acetone-dried rabbit muscle which, when added to myosin A, changed its properties to myosin B. In this reaction this viscous protein, which was called actin, combined with myosin to form a complex (aetomyosin), the most important property of which is contractility (2). Straub's most interesting observation was that this viscous protein can be obtained, under specific conditions, in a non-viscous form, and that this protein solution becomes very viscous upon the addition of salts (3). He considered this change to be a polymerization of the globular actin to a fibrous form (3). Electron microscope investigations of Jakus and Hall (4) actually demonstrated the formation of long continuous fibers. Rozsa, Szent-GySrgyi, and Wyckoff (5) have shown with the aid of the electron microscope that the change of globular actin into the fibrous form takes place in two steps. First, the small globules (molecular weight ,~ 70,000) unite into large globular units (molecular weight--~ 1,500,000). Then, these units unite to form threads. Thus actin, besides being a very important contractile constituent of muscle fiber, constitutes an interesting example of protein polymerization.In this paper experiments are described which show that adenosine triphosphate is involved in the polymerization of actin. It has recently come to our attention that Straub (6) 1 also found that ATP is instrumental in bringing about actin polymerization.