Murine antibody responses to various species of insulin are under major histocompatibility complex-linked Ir gene control. Beef insulin differs from pork insulin by only two amino acids in the A-chain loop, yet strain C57BL/10 (B10) mice produce insulin-specific antibodies after immunization with beef insulin and fail to produce antibody after stimulation with pork insulin. Nevertheless, pork insulin primes helper T cells in B10 mice that can be demonstrated if insulin-specific Lyt-1-, -2+ suppressor T cells are removed. Not only do the pork insulin-primed helper and suppressor T cells cross-react with autologous insulin, but also rat insulin (the amino acid sequence of which is identical to mouse insulin) elicits functionally identical helper and suppressor T cells. In this report, we demonstrate that in B10 mice the frequency of helper T cells stimulated by pork insulin is equivalent to that stimulated by beef insulin and that helper T-cell clones induced by beef and pork insulin are major histocompatibility complex-restricted T cells that proliferate, produce lymphokines, and provide helper activity after activation. These helper T-cell clones exhibit different antigenic fine specificities: beef insulin-induced clones respond to beef insulin but not pork or autologous insulin, whereas pork insulin-induced clones cross-react with all species of insulin tested, including rat insulin. In addition, the helper activity of cloned pork insulin-specific T cells is abrogated by pork insulin-primed suppressor T cells. These data support the hypotheses that Ir gene control of antibody responses to certain antigens involves mechanisms used for maintenance of self-tolerance.