The quantity of myosin, actin, and actin-binding protein (ABP) in platelets and platelet fractions of 5 patients with Glanzmann’s thrombasthenia were compared to those of normal individuals. No significant differences were observed between the average amounts of these contractile proteins in unfractionated platelets or platelet membrane obtained after osmotic shock. However, the amount of myosin was significantly decreased in the KC1 extract of thrombasthenic platelets ruptured by freezing and thawing, while its specific ATPase activity was increase