Actomyosin was purified from human blood platelets and used to form threads via extrusion. A sensitive tensiometer was employed to measure isometric tension and velocity of isotonic shortening of the threads in the presence of MgATP. Using fully phosphorylated myosin, we obtained values for maximum isometric tension (Po) and maximum velocity of contraction (V max) that were similar to those reported for threads composed of skeletal muscle actomyosin. Po was found to be directly proportional to the level of phosphorylation of the 20,000-dalton myosin light chain. We also studied the effect of phosphorylation on superprecipitation of platelet actomyosin. Fully phosphorylated myosin produced rapid clearing and superprecipitation, while myosin with a low level of bound phosphate underwent rapid clearing but did not superprecipitate. We have concluded from these results that: 1) the interaction between platelet actin and myosin produces tension and motion that is similar to that produced by skeletal muscle actin and myosin and 2) phosphorylation of the 20,000-dalton myosin light chain in important in controlling the production of force by platelet actin and myosin.