The in vitro effects of aztreonam on platelet aggregation were compared with those of cefotaxime, moxalactam, piperacillin, and carbenicillin. In addition, the in vivo effects of intravenously administered aztreonam on blood coagulation and platelet function were examined in 10 normal male volunteers in a randomized crossover study. In vitro, at concentrations of greater than 6.25 mM (2.7 mg/ml), aztreonam inhibited ADP-induced platelet aggregation in a dose-dependent manner. The effect was less than that produced by equimolar concentrations of cefotaxime, moxalactam, piperacillin, or carbenicillin. At all concentrations tested, aztreonam and cefotaxime inhibited epinephrine-induced aggregation least. All antibiotics inhibited collagen-induced aggregation, but only at inordinately high concentrations (25 mM). In vivo studies in 10 male subjects, randomly infused intravenously with 2 g of aztreonam or saline placebo every 6 h for 21 consecutive doses in a single-blind crossover study, revealed no evidence of bleeding or visible adverse side effects. Although plasma coagulation and platelet adhesion remained within normal limits in all subjects throughout the study, inhibition of ADP-induced platelet aggregation significantly (P less than 0.0001) increased on days 3 and 6, but still was below 40%. With the exception of one subject who had a mean template bleeding time of 7.3 min (normal, 2 to 7 min at 95% confidence limits) on day 6 of aztreonam administration, all volunteers exhibited bleeding times within the normal range. No abnormalities in platelet morphology were observed. Mean peak serum aztreonam concentrations on days 1 and 6 were 90.1 +/- 16.7 and 95.9 +/- 13.7 micrograms/ml, respectively; accumulation did not occur. Thus, in normal volunteers, aztreonam produced no significant recognizable abnormalities of hemostasis after 6 days of maximal recommended doses.