Idiotype analyses of plaque-forming cells using anti-idiotype antibody against TEPC-15 myeloma protein (anti-T 15 id) indicated that the proportion of plaque-forming cells producing anti-phosphorylcholine (PC) antibodies with idiotypic determinants of T15 id varied depending on the strain of mice (15-97%). However, treatment of neonates of those mouse strains with the anti-T 15 id antibody rendered them unresponsive to subsequent stimulation with PC-containing antigen (less than 7% of control response). The treatment of spleen cell cultures, derived from adult mice, with anti-T 15 id antibody resulted in complete suppression of T 15 id, although the suppression of the total anti-PC response was much less pronounced as compared to that induced in vivo. These results suggest that anti-T 15 id antibodies injected in the neonatal period may chronically inhibit the differentiation of B lymphocytes specific for PC. The lack of compensatory increases in the production of anti-PC antibodies bearing other idiotypes in T 15 id-suppressed mice does not appear to be related to the clonal proportion of cells producing anti-PC with non-T 15 id. This tolerance induction by anti-idiotype antibody appears to be unique to the nature of clonal differentiation of anti-PC-producing lymphocytes.