The distribution of idiotype-positive plaques in mice of various strains injected with T-independent (TI) and T-dependent conjugates of p-azobenzenearsonate (Ars) has been investigated. With maturation of the responses there is a progressive loss of dominance of the major cross-reactive idiotype (CRI) in A/J and C.AL-20 mice. The extremes are represented by the predominantly CRI+ IgM plaque response to a single injection of the TI antigen, Ars-coupled Brucella organisms (Ars-Br), and the low frequency of CRI+ plaques in the polyisotypic response elicited by Ars-KLH (keyhole limpet hemocyanin) in adjuvant. The secondary response to Ars-Br was characterized by an intermediate display of idiotype-positive plaques in IgM, the IgG subclasses and IgA. Detailed analyses of the distribution of CRI+ plaques between isotypes in 60 mice hyperimmunized with Ars-KLH revealed that the expression of idiotype in any of the IgG subclasses and IgA was constant and independent. This is interpreted to indicate random assembly of VH with C gamma and C alpha genes during the switch. BALB/c and C3H mice also express a predominant idiotype during primary and secondary responses to Ars-Br as detected by our heterologous anti-A/J CRI serum. These strains only very rarely maintain the idiotype after hyperimmunization with Ars-KLH. We conclude that hyperimmune responses which probably involve multiple modes of regulation give a poor indication of the germ-line V gene repertoire.