Suppressor T-cell function activated by preincubation of peripheral blood mononuclear cells with concanavalin A (Con A) is mediated by the human lymphocyte subset which reacts with monoclonal antibody OKT5 8 (E. L. Reinherz, P. C. Kung, G. Goldsteing, and S. F. Schlossman, J. Immunol. 124, 1243, 1980; E. L. Reinherz and S. F. Schlossman, Cell 19, 821, 1980). Proliferative responses stimulated by mitogenic lectins involve T cells of both the OKT4 and OKT8 phenotypes (E. L. Reinherz and S. F. Schlossman, N. Engl. J. Med. 303, 370, 1980). As part of a study of immunologic parameters in patients with alopecia areata and concurrent normal controls we analyzed the results of functional responses of the subjects' T cells (mitogen proliferation and ability to generate Con A-activated suppression) according to the proportions of OKT4 (helper/inducer) and OKT8 (suppressor/cytotoxic) cells quantitated at the time of study ( OKT4 OKT8 ratios ranging from 0.53 to 3.55). Absolute total T-cell counts and lectin-stimulated proliferative responses (phytohemagglutinin, Con A) were similar for patient and control groups. Patients with alopecia areata, and particularly those who demonstrated regrowth of hair at the time that immunologic studies were performed, generated increased Con A-activated suppression in conjugation with an increase in the proportions of peripheral OKT8 cells. For all subjects studied, OKT4 OKT8 ≤ 1.5 correlated with Con A-activated suppression > 50% (P = 0.0006), and relatively decreased PHA and Con A proliferative responses (P < 0.01, P = 0.01, respectively). These findings suggest that functional responses of peripheral lymphocytes reflect the proportional distribution of peripheral immunoregulatory subsets among healthy individuals as well as patients with alopecia areata.