The effects have been studied of Pluronic F-68 at 0.04% (w/v) on platelet aggregation in hirudin (50 micrograms ml-1)-anticoagulated, human whole blood in vitro in response to the following aggregation agonists: (i) phorbol 12-myristate 13-acetate (PMA; 0.05, 0.1 or 0.15 microgram ml-1), (ii) collagen (0.125, 0.25 or 0.5 microgram ml-1), or (iii) ristocetin (0.3, 0.6 or 1.2 micrograms ml-1). Pluronic F-68 significantly (P < 0.05) inhibited platelet aggregation that followed the addition of all agonists at their lowest concentration tested. Pluronic F-68 had markedly less pronounced inhibitory effects on the platelet aggregation that occurred in response to 0.15 microgram ml-1 PMA, where the mean % aggregation after 8 min was 67% of control (P < 0.05). Pluronic F-68 did not alter platelet aggregation in blood treated with 0.25 or 0.5 microgram ml-1 of collagen.