To determine whether the hypoprothrombinemia associated with antibiotics containing a 1-methyl-5-thiotetrazole (MTT) group is a result of the presence of the MTT group, rats were maintained on a vitamin K-deficient diet for 10 days and then received either intravenous moxalactam or cefotaxime or oral MTT for two additional days. MTT and moxalactam, which contains the MTT group, prolonged prothrombin time. Cefotaxime, which lacks the MTT group, had no effect.