The use of a number of antibiotics which contain an N-methyl-thiotetrazole (NMTT) side chain has been reported to be associated with an increased incidence of hypoprothrombinemia. The suggested role of NMTT as an inhibitor of the liver microsomal vitamin K-dependent carboxylase has been investigated. In standard incubations, NMTT had no effect on carboxylation when vitamin KH2 was a substrate but was a weak inhibitor when [vitamin K + NADH] was a substrate. Microsomal vitamin K reductases, however, were not inhibited by NMTT. Preincubation of the incubation mixture with NADH and NMTT resulted in inhibition of carboxylase activity when either vitamin KH2 or [vitamin K + NADH] was the substrate. A fraction of the microsomal membrane which was not readily solubilized by dilute detergent protected the enzyme from this inhibition. The data suggest that NMTT is metabolized to an active inhibitor or is able to covalently inactivate the enzyme in the presence of NMTT. The vitamin K responsiveness of the clinically observed hypoprothrombinemia suggests that it is not related to this in vitro inhibition of the vitamin K-dependent carboxylase.