The effects of inhibitors of mitosis, energy metabolism and protein synthesis on clot retraction were investigated. The results show that (1) Incubation of colchicine (0-01-0-1 mM) with platelet-rich plasma (PRP) inhibits the subsequent retraction of clots derived from diluted PRP. (2) Inhibition of clot retraction by high concentrations of colchicine (up to 40 mM) can be overcome by increasing the platelet concentration in the system. (3) Incubation of clots in colchicine or 80% D2O solutions inhibits their retraction. Exposure of partially retracted clots to these agents is without effect. (4) Hydrostatic pressure retards clot retraction. (5) Incubation of PRP with either 2-deoxy-D-glucose or antimycin alone does not affect clot retraction, but a combination of these agents is inhibitory. (6) Clot retraction is not inhibited by puromycin or cycloheximide. (7) Platelets in retracting clots have constricted regions containing microfilaments and pseudopods containing microtubules. Fibrin strands are progressively condensed around the constricted regions as retraction advances. (8) The development of platelet constriction, platelet pseudopods and the intracellular microfilaments are delayed in colchicinized clots, corresponding to the retardation of retraction. Following the initial delay of retraction colchicinized clots, like controls, show condensation of fibrin strands adjacent to these constricted areas of platelets containing microfilaments. The formation of pseudopods is impaired and no microtubules are found in platelets in the presence of colchicine. The above results suggest that the thrombin-induced platelet contraction during clot retraction is a coordinated movement, which, under optimal conditions involves both microtubules and microfilaments. The contraction of microfilaments produces the constriction of platelets and brings about clot retraction by reducing the angle between fibrin strands. Platelet microtubules are related to the development of pseudopods and play a supplementary role in facilitating microfilament-mediated cellular constriction. The similarities between platelet contraction and cellular motility in mitosis is discussed.