A new procedure of purification of actin from human blood platelets was used. This method starting from acetone powder of whole platelets gives a much higher yield than the one previously described (actin I) (Landon et al. (1977) Eur. J. Biochem., 81, 571-577). This actin II preparation has the same reduced viscosity as skeletal muscle actin, while the reduced viscosity of actin I preparation is about 1/10 of this value. Moreover actin I has the form of very short filaments as shown by electron microscopy. After an extra step of purification actin I, when polymerized, acquired a high reduced viscosity. We confirmed that platelet and sarcomeric actins are similar in their polymerization properties and their ability to activate muscular myosin. A circular dichroism study showed that the overall conformation of both actins are similar, but the environment of their aromatic chromophores is different.