Overwhelming infection with gram-negative bacteremia has become the most serious nosocomial infection in compromised patients. Because gram-negative bacteria share a common core lipopolysaccharide, we tried to develop a single vaccine or antiserum that might control these infections regardless of species. We used a mutant of Escherichia coli 0111 (J5) deficient in uridine diphosphate-galactose (UDP-GAL) epimerase and thus unable to attach "0" side chains, so that core lipopolysaccharide was exposed. A vaccine composed of this mutant produced antibody that gave broad protection against lethal infections by different gram-negative bacteria in immunosuppressed animals. The J5 vaccine protected against 98 percent lethal doses of Pseudomonas aeruginosa, and J5 antiserum improved survival tenfold in animals dying of Esch. coli, Klebsiella and Pseudomonas bacteremia. The protection with vaccine or prophylactic antiserum was undiminished in animals challenged six weeks after immunization. Encouraged by these results, we conducted a double-blind trial in patients with gram-negative bacteremia. In those given J5 antiserum, the mortality rate was cut in half and survival from deep shock increased from 28 percent to 82 percent. Because of these preliminary results in 136 patients, the study has been extended to 300 patients and the double blind code will be examined again to see if the early favorable results are confirmed and extended.