Vaccines prepared from unheated and boiled, acetone-precipitated Salmonella minnesota R595 (Re chemotype mutant) were administered subcutaneously to 122 healthy volunteers. Titers of antibody to Re lipopolysaccharide, the basal core structure of endotoxin, as measured by indirect hemagglutination, rose in a dose-responsive fashion after immunization. Febrile reactions, usually mild, occurred after 7% of injections with the highest doses (2.0 and 3.0 x 10(10) organisms), and mild local soreness and tenderness were noted after approximately one-third of injections. Passive immunization of mice with sera from immunized subjects demonstrated that protective activity against challenge with both heterologous, viable gram-negative bacilli and endotoxin developed. Although measuring serum protective activity, developing a serological assay that correlates with protective activity, and potential vaccine toxicity remain problems, immunization of humans with the Re mutant results in serum protective activity against endotoxin and viable bacilli and has the potential for clinical usefulness.