Human blood platelets were subjected to osmotic shock, brief sonication, pressure homogenization, or treatment with adenosine diphosphate (ADP). These procedures demonstrated an abundance of cytoplasmic microfibrils. The fibrils resembled those found on electron microscopy of partially purified thrombosthenin, the actomyosin-like protein isolated from platelets, and they also appeared to resemble the myofilaments of smooth muscle. Similar fibrils were not found in leukocytes studied under identical conditions. Treatment with colchicine (2 x 10(-5) mole/liter) resulted in the disappearance of microtubules but did not affect the morphology of the microfibrils or interfere with platelet-dependent clot retraction. Thus, microfibrils rather than microtubules may represent the morphologic counterpart of the contractile protein. Brief osmotic shock at low temperature or treatment with 10(-4) M ADP caused the marginal band of microtubules to be replaced by a bundle of intertwining microfibrils. The apparent inter-conversion of microtubules and microfibrils under a variety of conditions led to the hypothesis that fibrils and tubules consist of similar subunits whose degree of polymerization might be dependent on local cytoplasmic forces. Furthermore, on the basis of these observations, it is postulated that the contractile properties of the cells may be vested in the microfibrils, whereas the tubules may serve to maintain the highly asymmetric shape characteristic of circulating and irreversibly aggregated platelets.