It is well-known that immunization of guinea pigs with hapten-protein conjugates induces concomitant production of IgGl and IgG2 anti-hapten antibodies. However, the synthesis of antibody to 2, 4-dinitrophenyl groups (DNP) was found to be restricted to one of the IgG antibodies (IgG2 antibody) when guinea pigs were immunized through repeated intraperitoneal injections of 0.1 mg of 2, 4-dinitrophenylated Escherichia coli (DNP-E. coli). This selective induction of IgG2 anti-DNP antibody formation occurred both in the presence and absence of Freund's adjuvants, whereas a trace of IgGl anti-DNP antibody was produced concomitantly on increasing the immunizing dose of DNP-E. coli (3.0 mg) in Freund's complete adjuvant (FCA).