Comparative studies on pathogenicity between Yersinia enterocolitica and Yersinia pseudotuberculosis were performed using experimental infection systems in vivo and in vitro. All of thestra ins of both species successfully produced experimental enterocolitis in rabbits although the severity varied with the strains challenged. The changes were characterized by granulomatous lesions with necrobiotic centers in reticuloendothelial tissues of the intestine, mesenteric lymph nodes, liver and spleen. These strains uniformly had the ability to penetrate HeLa cells and to survive or multiply within cultured rabbit peritoneal macrophages. In addition, in infections with strain TP-2 or PST-III of Y. pseudotuberculosis, catarrhal inflammation all over the small intestine and/or focal necrosis and parenchymatous degeneration in the liver were observed, along with the granulomatous lesions. These strains, at the same time, exhibited cytotoxic effects on the cultured cells. The pathogenic factors of Y. enterocolitica are discussed in comparison with those of Y. pseudotuberculosis.