A troponin I-like factor has been purified from pig platelet by G150 Sephadex filtration of a low ionic strength extract, acidification at pH 4.2, ion exchange on DE-52 cellulose, and affinity chromatography on calmodulin-Sepharose. This protein (M r 17000), together with pig brain calmodulin and platelet tropomyosin, is able to participate to the reconstitution in vitro of a thin filament-like complex which modulates with 55% calcium sensitivity 1 1Calcium sensitivity = [1 - (ATPase EGTA/ATPase calcium)] x 100 the platelet actin-activated Mg 2 + -dependent ATPase activity of rabbit skeletal muscle myosin.