Serologic and chemical studies have suggested that immunoglobulin structural genes are under nonallelic genetic control. The transient appearance of unexpected (i.e., latent) allotypes in serum further suggests that cellular control mechanisms may be operative. We report here the results of our efforts to induce the expression of latent allotypes by cultured rabbit splenocytes. The protein A plaque assay facilitated with various anti-allotypic antisera was used to enumerate allotype-specific plaque-forming cells. When splenocytes from a b4 b4 rabbit are cultured in the presence of anti-b4 alloantisera, the number of b4 PFC (i.e., nominal) is suppressed and the number of PFC expressing the latent allotype is increased. The specificity of the induced latent PFC is dependent upon the source and allotypic mosaic of the antiserum added to the cultures. Our results suggest that lymphocytes committed to latent allotypes are normally suppressed. Activation by the appropriate alloantiserum directed against the nominal allotype may relieve a cell-mediated suppressive mechanism.