This paper documents the reversible appearance of high-affinity complexes of profilin and gelsolin with actin in extracts of platelets undergoing activation and actin assembly. Sepharose beads coupled to either monoclonal anti-gelsolin antibodies or to polyproline were used to extract gelsolin and profilin, respectively, from EGTA-containing platelet extracts and determine the proportion of these molecules bound to actin with sufficient affinity to withstand dilution (high-affinity complexes). Resting platelets (incubated for 30 min at 37 degrees C after gel filtration) contained nearly no high-affinity actin/gelsolin or actin/profilin complexes. Thrombin, within seconds, caused quantitative conversion of platelet profilin and gelsolin to high-affinity complexes with actin, but these complexes were not present 5 min after stimulation. The calcium-dependent actin filament-severing activity of platelet extracts, a function of free gelsolin, fell in concert with the formation of EGTA-stable actin/gelsolin complexes, and rose when the adsorption experiments indicated that free gelsolin was restored. The dissociation of high-affinity complexes was temporally correlated with the accumulation of actin in the Triton-insoluble cytoskeleton.