To determine if platelet dysfunction caused by high doses of penicillin compounds is of practical importance in patients with additional haemostatic defects perioperatively, a study was made of patients undergoing open-heart surgery. They were randomly assigned to prophylactic treatment with ampicillin 8 g plus cloxacillin 4 g daily for three days, or with cephalothin 8 g daily for three days. Fifty patients in each group were evaluated. The median bleeding time preoperatively and on days 1 and 4 postoperatively did not differ between the groups. The bleeding time was prolonged beyond the normal range in eight patients of the ampicillin/cloxacillin group and in three of the cephalothin group (p less than 0.05). Prolonged bleeding time was not associated with lower platelet count or greater blood loss. The total blood loss and the amounts of transfused blood, platelets and cryoprecipitate were all greater in the ampicillin/cloxacillin group, but the difference was not statistically significant. Combined use of ampicillin and cloxacillin in open-heart surgery is associated with increased bleeding, but the increase is without practical importance.