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INTRODUCTIOKThis work is concerned with the study of an extract which, like muscle actomyosin, markedly changes in viscosity in the presence of low concentrations of adenosine triphosphate. This extract which was obtained from the plasmodium of the myxomycete Pliysarzon polycephalzcm, was studied in the hope that it may give insight in the mechanism by which unspecialized or primitive tissue is able to convert chemical energy into mechanical work.One of the fundamental qualities of living matter is the ability to perform mechanical work. This characteristic has reached its evolutionary culmination in the muscle cell. Indeed the capacity for work in this highly specialized machine is so well developed that one is apt to ignore it in other cells.And yet all cells, a t least a t some stage of their life history, engage in mechanical work. The beat of the cilium and flagelluin, the streaming of cytoplasm, the movement of chromosomes, the movement of amoeboid cells, the gliding of diatoms,