1. A procedure has been developed for the preparation of an active concentrate from the myxomycete, Physarum polycephalum. This concentrate responds with a lowered viscosity to the addition of small amounts of ATP. The preparation recovers in viscosity, and the process may be repeated.

2. In the most active concentrates, 75 per cent of the non-dialyzable material moves as a single boundary both in the descending limb in electrophoresis and in the ultracentrifuge. It contains about 10 per cent ribonucleic acid, which is at least in part reversibly bound to the protein.

3. The active material has been designated myxomyosin because of its origin and its similarity to actomyosin in ATP response.

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