One hundred fifty-six hospitalized adult patients treated with ticarcillin, piperacillin, mezlocillin, or cefotaxime (control) wereprospectively observed for determination of frequencies of platelet dysfuntion and bleeding. Increases in bleeding times (⩾5 min above pretreatment values) occurred in 73% of patients receiving ticarcillin, 43% receiving piperacillin, 25% receiving mezlocillin, and 17% receiving cefotaxime (P < .0001); chemotherapy, thrombocytopenia, age of ⩾60 years, dose of ⩾12 g per day, and duration of treatment of six or more days were significant covariables. Significant bleeding occurred in 34% of patients treated with ticarcillin, 17% with piperacillin, 2% with mezlocillin, and 5% with cefotaxime (P = .0005); chemotherapy, thrombocytopenia, primary marrow disorders affecting platelet function, prolonged prothrombin time, and azotemia were significant covariables. Bleeding was associated with an elevated pretreatment bleeding time, an increase in bleeding time during treatment, and the maximal observed bleeding time during treatment (P < .0001).