The response of BALB/c mice to Pneumococcus pneumoniae is dominated by a single idiotype defined by the phosphorylcholine-binding myeloma protein TEPC 15 (T15). Injection of newborn BALB/c mice with antibodies to the T15 idiotype will lead to suppression of this idiotype. Such mice will respond to the antigen Pneumococcus, but with idiotypically different anti-phosphorylcholine antibodies. In an attempt to further characterize the effect of anti-idiotypic antibodies on induction of tolerance, three monoclonal antibodies specific for different T15 idiotopes were used. Although these antibodies belonged to the mouse immunoglobulin classes, IgG1 or IgG2 alpha, and were derived from different mouse strains, in all cases, a long lasting suppression could be induced. The data show that injection of idiotope-specific antibodies will suppress the T15 idiotype, but not necessarily the idiotope itself. The results are discussed in terms of the mechanism which will lead to the suppression of a dominating idiotype in the newborn mouse.