Several new beta-lactam antibiotics impair normal hemostasis. Hypoprothrombinemia has occurred frequently with cephalosporins that possess a methylthiotetrazole substitution (cefamandole, moxalactam, and cefoperazone). The incidence ranges from 4% to 68%, and the risk is greatest in debilitated patients with cancer, intra-abdominal infection, or renal failure. Impaired platelet function caused by perturbation of agonist receptors on the platelet surface has occurred primarily with beta-lactam antibiotics having an alpha-carboxyl substitution (moxalactam, carbenicillin, and ticarcillin). These antibiotics often cause the template bleeding time to be markedly prolonged (greater than 20 minutes). Acylureidopenicillins, which lack the alpha-carboxyl marker, impair platelet function less frequently and only modestly prolong the bleeding time. If serious hemorrhage occurs, hypoprothrombinemia associated with methylthiotetrazole-substituted cephalosporins should be treated with fresh frozen plasma. Likewise, dangerous bleeding due to impaired platelet aggregation requires treatment with platelet concentrates.