Human Yersinia enterocolitica septicemia is an uncommon condition. Four new cases are reported here and a review is made of 51 others taken from medical literature. Septicemia caused by this microorganism occurs more frequently in the young and in the elderly, and usually involves patients havig previous liver or blood disorders, diabetes mellitus, and other debilitating diseases. Clinically it is indistinguishable from sepsis caused by other organisms of Enterobacteriaceae, but it is important that the clinician bear its existence in mind, since Yersinia enterocolitica strains are usually resistant to beta-lactam antibiotics, whereas they are susceptible to the aminoglycosides and co-trimoxazole, among others. Susceptibilities in the blood isolates from our patients, and in another ten fecal isolates from eight other patients showed the previously described pattern. Our isolates, however, were all susceptible to the new cephalosporins, cefamandole and cefoxitin, and to the experimental ones, HR-756, T-1551, and Ly-127.935.