Blood donated to the Natal Blood Transfusion Service was screened by an enzyme-linked immunosorbent assay (ELISA) for anti-lipopolysaccharide (anti-LPS) antibodies. Plasma units with high concentrations (greater than 40 micrograms/ml) of anti-LPS IgG were pooled and fractionated to obtain a gamma globulin (lot LG-1). The binding of LG-1 antibodies to LPS prepared from 14 bacterial species and strains was found to be the highest to LPS from Shigella flexneri, Salmonella abortus equi and Salmonella typhimurium and intermediate with Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella enteritidis and Escherichia coli 026:B6. Differential absorption experiments showed that LG-1 contained a mixture of specific and cross-reacting antibodies. A large proportion of antibodies binding to Sh. flexneri LPS were mainly specific, while those binding to S. typhimurium and the other Salmonella species tested were largely cross-reactive. There was little correlation between the spectrum of activity of the LG-1 antibodies and the incidence of gram-negative bacteria in blood cultures taken from hospital patients in an area covered by the Transfusion Service. Mice treated with LG-1 prior to inoculation with P. aeruginosa were significantly protected against morbidity and mortality compared to controls.