Cephalosporin antibiotics with N-methyl-thio-tetrazole (NMTT) side chains have been known to be associated with the development of hypoprothrombinemia. However, it has not been fully established whether these symptoms are induced by an inhibition of vitamin K production by intestinal microorganisms or by an inhibitory action of these antibiotics on endogenous vitamin K metabolism. Therefore, an attempt has been made to clarify the above-mentioned ambiguity by using germfree mice in which primary vitamin K deficiency can be established within a short experimental period. Germfree (GF) and conventional (CV) ICR male mice, 8-13 weeks old were used in this experiment. Vitamin K deficient (Def) and menaquinone-4 supplemented diet (MK-4) were fed to the mice in both rearing conditions. In the antibiotic-treated group, sodium latamoxef (LMOX, 300 mg/kg B.W./day) was intraperitoneally administered once a day, and in the control group the same volume of saline (Saline) was administered. Severe vitamin K deficient symptoms were observed in the GF-K-Def-LMOX group, and both prothrombin time (PT) and activated-partial thromboplastin time (APTT) values were prolonged on the 8th day of the experimental period compared with the GF-K-Def-Saline group. Furthermore the mortality rate of GF-K-Def-LMOX group was comparatively higher than that of the Saline group. This study has provided evidence that vitamin K deficiency is amplified by an administration of LMOX even in the absence of intestinal flora.