Concurrent injection of monophosphoryl lipid A (MPL) in saline or as an oil-in-water emulsion enhanced both the primary and secondary serum antibody responses to the capsular polysaccharide (CP) components of seven conjugates: the enhanced responses were Ag-specific. In contrast, MPL did not enhance the serum antibody response to five of the six unconjugated CP. MPL and trehalose dimycolate injected concurrently with the unconjugated Vi CP of Salmonella typhi (Vi) enhanced the serum antibody response to that Ag. MPL further enhanced the Vi antibody levels when injected with conjugates of this CP. The serum antibody responses to Pseudomonas aeruginosa exotoxin A, used as the carrier protein for the Staphylococcus aureus types 5 and 8 conjugates, were also enhanced by MPL. MPL in oil-in-water emulsion was generally more effective than when administered in saline.