AbstractThe primary immune response to dinitrophenylated mouse serum albumin (DNP8MSA, DNP32MSA) and dinitrophenylated bovine γ‐globulin (DNP12BGG) was investigated in different mouse strains. It was found that DNP12BGG elicits specific antibodies against 1) DNP, 2) new antigenic determinants (NAD) introduced by the hapten coupling reaction and 3) BGG. DNP8MSA induces specific antibodies against 1) DNP and 2) NAD, and DNP32MSA induces anti‐DNP antibodies only. Further evidence for regarding the haptenic groups and the NAD as distinct antigenic specificities was obtained in mice high or low responders to the haptens. These mice respond equally well to the NAD.The amount, class distribution and avidity of antibodies produced against DNP8MSA, DNP32MSA and DNP12BGG were studied and the results will be discussed in relation to the cooperation hypoth