Between mid-1970 and April 1, 1971, Enterobacter cloacae or E. agglomerans septicemia developed in 378 patients in 25 American hospitals while they were receiving intravenous products manufactured by one company. Each of the hospitals noted a marked increase in the Incidence of such septicemia during this period. Enterobacter agglomerans (formerly designated Erwinia, herbicola-lathyri group) was better known as a plant pathogen and had been a human blood pathogen only rarely in the past. Septicemia caused by E. cloacae had also been uncommon. Clinical manifestations were indistinguishable from those of patients with other nosocomial gram negative sepsis except that most affected patients had no predisposing diseases or other sites of infection, half had infusion phlebitis, and appropriate antimicrobial therapy was clinically ineffective until the infusion was removed. These infections contributed to the death of 40 patients (13.4 per cent of cases), more than half of whom had underlying fatal diseases. The manufacturer was using a new Elastomer liner in the threaded cap of bottles of parenteral solutions; 39.9 per cent of 1,007 caps tested contained microorganisms, 8.1 per cent E. cloacae or E. agglomerans. These organisms were isolated from 0.6 per cent of cultures of fluid from stock supplies of bottles with Elastomer-lined closures and were recovered from intravenous fluid being administered lo septicemic patients. Organisms were easily transferred from the closures to the fluid by shaking the bottle. Outbreaks were terminated within individual hospitals only by discontinuing the use of the manufacturer's infusion products and nationwide by a total product recall. This epidemic focuses attention on infusion fluid as a vehicle of infection, the value of surveillance of nosocomial infections, the need for more vigorous preventive measures, and the danger of using conventional screw-cap closures with products that must remain sterile.