Normal volunteers and patients were studied to determine the relative importance of NMTT and patient risk factors in the production of hypoproth-rombinemia. The normal volunteers demonstrated in vivo NMTT production, but the order of magnitude (cefoperazone, moxalactam, and cefotetan in descending order) was different from the usual order of clinical risk. In patients, there was not a NMTT-concentration-versus-effect relationship. Patients who were vitamin K deficient were more sensitive to lower NMTT concentrations than those with normal vitamin K status. In surveillance studies, NMTT-containing antibiotics were no more frequently associated with hypoprothrombinemia or bleeding than antibiotics that lack this moiety.