Lymphocyte subpopulations, total T cells, T helper and T suppressor subpopulations as identified by monoclonal antibody and functional assays of suppressor cells using concanavalin A (Con A) were studied in 20 measles patients and compared to matched controls. Results were also related to severity of disease. Mononuclear cell (MNC) pokeweed mitogen (PWM) stimulation was also assessed. Severity of measles was assessed by lymphopenia, serum antibody and C3 levels and extent of pneumonia. T lymphopenia in patients was due to a decrease in OKT4+ cells and OKT8+ cells as compared to controls with the former being more severely affected. Patients with severe measles had a more profound reduction in both subsets (OKT4+ 356 +/- 65 cells/microliters mean +/- SEM; OKT8+ 466 +/- 41 cells/microliter) than those with mild disease (975 +/- 199 cells/microliters; 1,473 +/- 242 cells/microliters; p = 0.0432; 0.0038 respectively). Patients with severe depletion of OKT4+ cells had raised levels of C3 (an index of poor prognosis). Suppressor cell activity was unaffected by measles. MNC PWM stimulation was lower in patients than controls. No correlation was detected between numerical and functional assays of suppression although there was a significant correlation between PWM stimulation and OKT4+ cell numbers in the control group (p = 0.0407).