Carbenicillin (CB-PC), sulphobenzylpenicillin (SB-PC), ampicillin (AB-PC), benzylpenicillin (PCG), cefazolin (CEZ), cephalothin (CET) and cephaloridine (CER) were used in vitro for the experiments. Recalcification time was prolonged by AB-PC, SB-PC, CB-PC and CER at the 10(-2) and 10(-3) M level. Prothrombin time (PT) was prolonged by AB-PC, CEZ, CET and CER at the 10(-2) M level. Plasma antithrombin activity was not affected by any antibiotics tested at the 10(-2) and 10(-3) M level. The platelet thromboplastic function of the human platelet suspension and free platelet factor III were reduced more than 40% by all antibiotics used except for PCG at the 10(-3) M level compared with controls, showing no significant difference in the degree of reduction between these two samples. When antibiotics were preincubated with albumin solution, the reduction of platelet thromboplastic function was greatly inhibited except with CB-PC and CER. Factors of bleeding caused by intramuscular injections of the antibiotics are discussed.