Antibodies specific for the idiotypes of B10 anti-(T,G)-A-L antibodies (anti-Id) induced in vivo (T,G)-A-L-primed lymphocytes to secrete anti-(T,G)-A-L antibodies in vitro in the absence of antigen. Responses to anti-Id were quantitatively and qualitatively similar to responses to antigen. Responses were specific in that: (a) only lymphocytes primed in vivo with (T,G)-A-L (but not other antigens or unprimed cells) were inducible; (b) only anti-Id (but not a variety of control antibodies) induced responses; and (c) only anti-(T,G)-A-L antibodies were secreted. Antigen-primed T lymphocytes mixed with unprimed B plus accessory cells also responded to anti-Id and the phenotype of the responding T cells was Lyt-1+, Lyt-2-. Supernatants obtained from antigen-primed T cells incubated for 3 d with anti-Id (but not when incubated with controls) provided help to unprimed B plus accessory cells in the presence of anti-Id, thus providing direct evidence for induction of T lymphocyte helper function by anti-Id. In contrast to responses to (T,G)-A-L, responses to anti-Id did not appear to be regulated by H-2-linked Ir genes. The system described is a powerful tool for delineation of the mechanisms whereby antiidiotype antibodies affect lymphocyte function.