Either alone or in combination with antibodies having specificity for the carrier erythrocyte, TNP-ORBC were injected i.p. into CBA/J mice. Five days later, their spleens were removed and evaluated for TNP-specific PFC. The spleens from animals receiving 19S antibody (IgM) with carrier specificity displayed 3-4-fold more direct and indirect hapten-specific PFC than control animals receiving the TNP-erythrocyte conjugate only. Animals receiving 7S antibodies (IgG) with carrier specificity displayed very little change in their direct PFC response to the hapten. However, the indirect response to the hapten was suppressed as much as 16-fold by carrier specific IgG. Evaluation by haptenic inhibition of the relative avidity of the antibodies being secreted by these cells revealed the following: the relative avidity of antibodies secreted by indirect PFC in the spleens of animals receiving TNP-ORBC only was approximately 20-fold higher than antibodies secreted by the direct PFC. The 3-4-fold potentiation of the hapten-specific PFC response by carrier-specific IgM antibody did not result in a change in relative avidity of direct or indirect PFC. IgG with carrier specificity did not change the relative avidity of the antibodies secreted by direct PFC having specificity for the hapten. However, evaluation of the remaining PFC in spleens from animals whose indirect hapten-specific PFC response had been suppressed by carrier-specific IgG revealed that the remaining PFC had a lower avidity than indirect PFC from animals receiving TNP-ORBC only. In other words, carrier-specific IgG selectively induced suppression of high avidity hapten-specific IgG antibody secreting cells.