This study was undertaken to gain insight into the functional properties of the expanded suppressor/cytotoxic lymphocytes characteristically found in actinic reticuloid. Peripheral blood cells, either whole mononuclear cell populations or selectively enriched populations, were obtained from a patient with well-established actinic reticuloid. The patient had an expansion of circulating T lymphocytes expressing the suppressor/cytotoxic phenotype, i.e., CD3+, CD4-, CD8+. A proportion of these cells also expressed the Fc receptor for IgG. Functional studies, including pokeweed mitogen-driven immunoglobulin synthesis, mitogen and alloantigen response, natural killer and antibody-dependent cellular cytotoxicity were within normal ranges and suggested a polyclonal rather than monoclonal expansion. The present functional data extend previous phenotypic studies and support the hypothesis that a chronic reactive immunoregulatory disorder is involved in actinic reticuloid, as has been hypothesized for other T cell chronic proliferations.