Biological activities of monoclonal A/J antibodies to the T15 idiotype in BALB/c mice were compared to heterogeneous antibodies raised by conventional immunization procedures. Two monoclonal antibodies, AB1‐2 and GB4‐10, which are of the γ₁,ϰ class, appeared to have identical specificities by binding criteria and reacted similarly to conventional antibodies in their abilities to induce neonatal suppression, inhibit plaque‐forming cell induction by phosphorylcholine (PC) antigens and to inhibit specifically, anti‐PC plaque‐forming cells. However, in functional analyses of anti‐PC responses in various strains of mice, discrepancies were noted in the T15 responses as defined by monoclonal antibodies and conventional antisera. This heterogeneity was also observed in adult mice suppressed with the GB4‐10 monoclonal antibody. These animals eventually produced an anti‐PC response of AB1‐2 idiotype but lacking the GB4‐10 marker.

These results show that the T15 IgM anti‐PC response in BALB/c and other strains of mice is heterogeneous and probably consists of a family of clones. Particular clones can be precisely eliminated by the use of appropriate monoclonal antibodies, and the anti‐PC response that eventually recovers is still T15⁺ but lacking the suppressed clones.