The current investigations examine the antigenic relationship of E. coli J5 to other antigens of Enterobacteriaceae and the role of J5 antigen(s) in cross-protective immunity. Surface antigens of E. coli J5 were revealed to be very complex. A cell wall preparation of J5 contained, in addition to specific J5 antigen, Enterobacterial Common Antigen (ECA-Kunin), Re inner core and a new common antigen which cross-reacted with S. minnesota and S. typhimurium. Immunization with heat-killed cells of J5 resulted in an antibody response with multiple reactivities; at least two antibody specificities, ECA-Kunin and specific J5 antigen, were demonstrated. The protective effect of J5 was shown in mice passively immunized with J5 antiserum before challenge. It was observed that antiserum to E. coli J5 enhanced the in vivo clearance of bacteria in mice, increased survival upon lethal challenge, but did not prevent death from endotoxin. The specificity of protection was assessed by absorbing antisera to E. coli J5 with J5 LPS and with two mutants of S. typhimurium, with and without ECA-Kunin. A loss of protective activity after absorption of J5 antiserum with purified homologous LPS implied that J5 LPS was responsible for the observed protection. This study provides further evidence for support of the role of core glycolipids as protective immunogens in gram-negative infections