Hypoprothrombinemic changes were compared in rats fed various vitamin K-deficient diets. Changes such as prolongation of prothrombin time and activated partial thromboplastin time, decrease in plasma levels of prothrombin and clotting factor VII, and increase in plasma descarboxyprothrombin, appeared in rats maintained on vitamin K-deficient diet, but in rats on ordinary diet (vitamin K-sufficient diet). As the development of hypoprothrombinemia was not significanthly different among animals fed various vitamin K-deficient diets, the blood coagulation parameters were concluded to be regulated only by the vitamin K level. Following the development of hypoprothrombinemia, hemorrhaging in various organs was detected in vitamin K-deficient rats, with strain differences in the severity of hemorrhage ; Fischer and Wistar strains were more sensitive than the Sprague Dawley strain. Administration of a beta-lactam antibiotic, latamoxef (LMOX), to vitamin K-deficient rats led to enhancement of the hypoprothrombinemic conditions, but LMOX-associated changes in plasma enzyme levels were not detected.