A protein has been studied which spontaneously precipitates from stored fractions of platelet soluble phase prepared by density gradient centrifugation. It is rich in a Ca2+ ATPase activity which displays an activity/pH profile resembling that of skeletal muscle myosin. Adjustment of freshly prepared soluble phase fractions to 0.6 M with respect to KCl and dilution 1 in 3 results in the precipitations of a protein fraction with essentially the same enzymatic properties as the spontaneously precipitable protein. These two similar proteins represent between 9 and 13% of the soluble phase total protein and each account for almost the whole of divalent cation activated ATPase activity of the soluble phases from which they were derived. The Mg2+ ATPase activity is only about twice purified with respect to the soluble phase enzyme activity, but the Ca2+ ATPase shows a 10-13-fold enrichment. Synthetic actomyosins can be prepared from the two proteins by addition of either platelet or skeletal muscle actin. These show significant increases in Mg2+ ATPase at the most favourable combination ratios. The ratio between the yield of soluble phase protein obtained by dilution precipitation and the lactate dehydrogenase activity of the soluble phase remains constant under a wide range of homogenization and sonication conditions applied to the original whole platelet suspensions. This confirms our earlier view that the soluble phase is a valid intracellular compartment for a considerable proportion of the platelet contractile protein and that in the complex the myosin-like component predominates.

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