We have shown previously that CBA/J mice immunized with Candida albicans developed delayed hypersensitivity (DH) demonstrable with mannan (MAN) extracted from the same organism and that the intravenous (i.v.) injection of MAN prior to or during the immunization phase resulted in the suppression of the MAN-specific DH response. In this study, we demonstrate that MAN-induced suppression of DH is a T-lymphocyte-mediated phenomenon. Suppressor cells induced in vivo by the i.v. injection of MAN into naive mice 1 to 7 days prior to harvest were passaged through nylon wool, treated with various surface-specific antibodies and complement, and then injected i.v. into immunized syngeneic recipients. Enrichment of splenic T cells by passage over nylon wool and transfer of the nylon-wool-nonadherent populations to immunized recipient mice suppressed DH in a dose-dependent manner. Depletion of Thy+ or Lyt-2+ cells from nylon-wool-nonadherent populations regularly ablated the ability of such suspensions to transfer suppression. Treatment of the same transfer suspensions with anti-Lyt-1 had variable effects, suggesting that the surface density of the Lyt-1 antigen was not as constant from population to population as was the Lyt-2 antigen. In addition, C. albicans MAN-induced suppressor cells were able to suppress DH demonstrable with Candida tropicalis MAN in animals immunized with C. tropicalis. Suppression of DH by MAN in this model, therefore, is mediated by Thy+ Lyt-2+ lymphocytes.