The autologous mixed lymphocyte reaction (AMLR) and T-cell subsets defined with monoclonal antibodies were examined in the peripheral blood of six patients with chronic mucocutaneous candidiasis (CMC). The AMLR was deficient in four of six patients when compared with simultaneously studied healthy controls. These patients had either overt endocrinopathy or circulating autoantibodies. In two of three patients with low AMLR, after the depletion of OKT8+ T cells no enhancement in the AMLR was observed, demonstrating that the deficiency of the AMLR was not due to increased suppressor OKT8+ T-cell activity. However, the third patient demonstrated almost complete reconstitution of the AMLR response after such depletion, suggesting that OKT8+ suppressor T cells were responsible for decreased AMLR in this patient. Two patients had a low ratio of OKT4/OKT8 phenotype T cells. T cells with Tac antigen (that is, present on activated T cells) were increased in four of six patients. No correlation was observed between the deficiency of the AMLR and the proportions of T-cell subsets. This study demonstrates a deficiency of the AMLR in some patients with CMC which is associated with increased OKT8+ suppressor T-cell activity or with the functional deficiency of responder OKT4+ T cells.