Mice of the C.AL-20 strain, which express genes controlling CH regions of the AL/N strain on a BALB/c background, normally synthesize antibodies to the p-azophenylarsonate group (anti-Ar antibodies) with an idiotype characteristic of the A strain. The synthesis of the idiotype, as quantitated by a sensitive assay, can be completely inhibited by the transfer of leukocytes from BALB/c mice producing anti-Ar antibodies, which lack the idiotype. A number of control experiments show that the inhibition is not attributable to suppressor T cells and that the synergistic action of such cells is not required. The results indicate that B-cell dominance, mediated by secondary cells, can completely prevent the expression of unprimed cells with receptors of the same specificity. It is uncertain whether this effect is due entirely to selective capture of antigen by the secondary cells, or whether some type of active suppression by B cells is involved.