A histopathological study using enzyme immunoassay (EIA) was carried out in mice experimentally infected with Yersinia pseudotuberculosis IB to elucidate the role of V antigen in vivo. The presence of V antigen and bacterial cells in the lesions was revealed in various organs by EIA using anti-V antigen serum and anti-O antigen serum. There was a distinct difference in the degree of V antigen production among the organs examined. V antigen was massively produced in the liver, spleen and mesenteric lymph nodes associated with lesions and bacteria. In the intestine, however, V antigen was weakly produced in the lesions under the intestinal epithelium despite the massive presence of a bacterial colony. The detailed staining pattern suggested that V antigen might be formed surrounding the bacterial cells in the lesions of the liver, spleen and mesenteric lymph nodes. Yersinia pseudotuberculosis IB mutant lacking in both VW antigen and plasmid did not cause lesions in any organs. In addition, fecal excretion of the organisms was closely associated with the formation of lesions in the intestine.