3-(1-Methyl-2-hydroxyethyl)-benzophenone (F025) is a close structural analogue of ketoprofen, in which the carboxylic acid of ketoprofen is reduced to a carbinol moiety. The in vitro effects of F025 on platelet aggregation, cyclooxygenase and hypotonic NaCl-induced erythrocyte hemolysis were examined in comparison with those of ketoprofen. Although the inhibitory activity of F025 on collagen, arachidonic acid and ADP-induced platelet aggregation was between 2 and 20 times that of ketoprofen in healthy volunteers, guinea pigs, rabbits and rats, F025 had only one-tenth of the inhibitory action of ketoprofen on sheep seminal vesicle cyclooxygenase F025, but not ketoprofen, inhibited hypotonic NaCl-induced erythrocyte hemolysis in guinea pigs. The substantial antiplatelet activity of F025 may be due to membrane stabilization as well as to cyclooxygenase inhibition.