Effects of the protein phosphatase inhibitors, tautomycin and calyculin A on protein phosphorylation and cytoskeleton of human platelets. It has been discovered recently that many cytotoxic compounds isolated from a variety of sources are potent phosphatase inhibitors. Two of these, tautomycin (TM) and calyculin-A (CL-A) were applied to human platelets to investigate the role of protein phosphorylation on cytoskeletal structure and function. Exposure to 10 μM TM or 0.1μM CL-A induced marked morphological changes. The granules were centralized and surrounded by actin filaments, but there was no evidence of granule release. Myosin became more centralized, was occluded from the granulomere, but was not confined to the micro filament ring. These changes occurred without an increase in cytosolic Ca²⁺ concentrations, as determined by measurements with fura-2. TM and CL-A induced an overall increase in protein phosphorylation. Phosphorylation of the 20, 000 dalton light chain of myosin increased markedly and multiple phosphorylation sites were indicated. Cytoskeletons were prepared from control, thrombin- and TM-treated platelets, the latter prepared in the absence of external calcium. The major difference in protein composition was the increased content of myosin associated with the cytoskeleton from TM-treated platelets where the dominant phosphoprotein was the 20, 000 dalton light chain. These results suggest that myosin phosphorylation drives the initial shape changes, and via a contractile process results in the formation of the micro filament ring and centralization of granules.