The immunocompetence of the circulating lymphocytes of a patient with non-Hodgkin's lymphoma was evaluated before, during and after treatment. The immunologic features investigated were (a) the degree of blastogenic transformation induced by phytomitogens (phytohemagglutinin-M, concanavalin A and pokeweed mitogen); (b) the formation of E, EA and EAC rosettes; and (c) antibody-dependent, cell-mediated cytotoxicity. Prior to treatment the subnormal in vitro responsiveness of the cells and the very low concentrations of circulating immunoglobulins suggested immunoincompetence. A gradual shift towards normal in vitro values was observed during treatment, which corresponded to the clinical improvement in the patient's condition.