Septicemia is a rare but serious complication of infection with Yersinia enterocolitica (Y.e.). Seven cases of Y.e. septicemia are presented. Five of the patients had no underlying disease predisposing to septicemia. Five patients displayed recurrent episodes of septicemia, despite treatment with recommended doses of antibiotics to which the isolates were sensitive in vitro. One patient developed endocarditis which required surgical replacement of the aortic valve. Other clinical manifestations were arthritis, diverticulitis and pulmonary abscesses. The outcome was fatal to 3 elderly patients. The serological response to Y.e. was followed by tube agglutination and a diffusion-in-gel enzyme-linked immunosorbent assay. One patient, with a benign course of illness, had transient elevated Y.e. antibody titres, while the 3 cases with a protracted disease showed sustained antibody responses for 6-18 months. Blood isolates of Y.e. had ordinary virulence characteristics identical to fecal isolates and produced extracellular beta-lactamase. All isolates were sensitive in vitro to trimethoprim-sulfamethoxazole, mecillinam, piperacillin, cefotaxime, ceftazidime, chloramphenicol and gentamicin. The lowest MIC values were recorded for mecillinam. Full synergistic activity was demonstrated when mecillinam was combined with trimethoprim-sulfamethoxazole, cefuroxime or rifampicin.