Murine antibody responses to heterologous insulins are under H-2-linked immune response (Ir) gene control. We have found that the immune response to insulin in adjuvant can be inhibited by prior i.v. injection of soluble insulin. The effect of i.v. injection of insulin is antigen-specific and dose-dependent and requires the same doses of insulin that are immunogenic if administered with adjuvant. In addition, the inhibitory effect of soluble insulin is dependent upon the route of injection; if soluble insulin is injected i.p., the subsequent response to insulin in adjuvant is augmented rather than inhibited. Unresponsiveness requires at least 4 days after i.v. injection to develop and once induced, it is maintained for 4 wk or more. Unresponsiveness is caused by T cell, but not B cell, tolerance, and we have been unable to demonstrate any role for suppressor T cells in this unresponsiveness. More importantly, analysis of the ability of numerous insulin variants to induce unresponsiveness in several H-2(k) and H-2(b) strains of mice has demonstrated that only the variants that were immunogenic in a given strain when administered with adjuvant were able to cause tolerance. This report is, to our knowledge, the first describing that induction of helper T cell tolerance, like the induction of immunity, is controlled by H-2-linked Ir genes.