Previous work has shown that the injection of antiidiotope antibodies specific for idiotopes of the germline-encoded anti-(4-hydroxy-3-nitro-phenyl) acetyl (NP) antibody B1-8 enhanced or suppressed the expression of B1-8 idiotopes in subsequent humoral anti-NP responses, depending on the dose and perhaps also the isotype of the injected antibody. To formally answer the question of whether the isotype of an antiidiotope determines its effector function in this type of idiotypic control, we have performed regulatory experiments with isotype switch variants selected from two hybridomas secreting anti-B1-8 idiotopes of CBA (Ighj) and C57BL/6 (Ighb) origin. The antibodies of each variant family differ from each other only in the constant region of the heavy chain. The results show that, irrespective of whether an antiidiotope antibody belongs to the IgG1, IgG2b, IgG2a, or IgE class, a 10-ng dose enhances idiotope expression whereas a dose of 10 micrograms exerts a suppressive effect. It emerges from the present and parallel data that the expression of antibody V regions resembling idiotypically that of antibody B1-8 can be enhanced and suppressed by any of four antiidiotope antibodies that recognize distinct idiotopes on those V regions. This suggests that the initial step in the regulatory process induced by an antiidiotope is its binding to antibody V regions carrying the target idiotope. The antiidiotopes preferentially regulate the expression of antibodies that coexpress with the target idiotope other B1-8 idiotopes, despite the fact that some B1-8 idiotopes are also expressed independently of each other in anti-NP responses of idiotypically unmanipulated mice. This finding may reflect high affinity binding of the antiidiotopes to the target against which they were originally raised (i.e., antibody B1-8) or, more likely, a preferential recognition of B1-8-like V regions by regulatory T cells.