Bovine platelets of which phospholipids were labeled with [14C] arachidonate were stimulated by collagen. Omitting albumin in suspending medium suppressed aggregation and arachidonate liberation from phospholipids. Analysis of [14C] arachidonate metabolites released into medium revealed that release of 12S-hydroxyeicosatetraenoic acid (12-HETE) from stimulated platelets was dependent on the existence of albumin, while thromboxane B2 release was not. Moreover, exogenously added 12-HETE inhibited collagen-induced aggregation. These observations suggest that albumin potentiates aggregation by preventing intracellular accumulation of 12-HETE, and strengthen our previous finding that albumin in plasma is very essential for collagen-induced aggregation of platelets (Sekiya, F., Takagi, J., Kasahara, K., Inada, Y. and Saito, Y. (1988) Thrombos. Res. 50, 837-846.).