We studied binding of (125)I-labeled prothrombin to platelets in the presence of circulating endogenous lupus anticoagulant. Lupus anticoagulant modulated the interaction and increased affinity of prothrombin for calcium-independent binding sites on platelets. The number of these sites decreased, while the total number of calcium-dependent binding sites increased. Our results indicate that lupus anticoagulant plays a role in the binding of prothrombin to nonactivated platelets.