In general, immunization of guinea pigs with hapten-protein conjugates using the usual immunization techniques induces concomitant production of IgG1 and IgG2 anti-hapten antibodies. However, when lipopolysaccharides (LPS) of Escherichia colt were used as a carrier of 2, 4-dinitrophenyl residues (DNP) and injected intraperitoneally with Freund's incomplete adjuvant (FIA), anti-DNP antibody production was enhanced markedly as compared with bovine serum albumin (BSA). Furthermore, the DNP-LPS was found to induce preferential production of IgG2 anti-DNP antibody not accompanied by IgGi anti-DNP antibody.