The amount of profilactin in platelet extracts made in the absence of free Ca++ ions decreases and the amount of free profilin increases as a consequence of thrombin stimulation. This agrees with the proposed role of profilactin as a microfilament precursor in nonmuscle cells. Filamentous actin in extracts of unstimulated platelets appears partly in large aggregates that contain actin binding protein (ABP) and relatively few other proteins. After stimulation, the amounts of actin and ABP in the aggregates are increased and myosin is also included together with a few additional proteins. When the cells are lysed in the presence of Ca++, aggregation is drastically reduced. The data indicate that filamentous actin depolymerizes rapidly and recombines with available profilin, and that a Ca-specific interaction also occurs between actin and a new protein with molecular weight about 90,000.