Peripheral blood lymphocytes of thirty normal volunteers and fifty-two patients with systemic lupus erythematosus were fractionated using a discontinuous (5-30 percent) Ficoll gradient. Such fractionation permitted the isolation, identification of study of null cells, T cells and B cells. Patients with inactive SLE were found to have a cell distribution and responsiveness to PHA, Con A and Pokeweed mitogen (PWM) similar to controls. In contrast, patients with active SLE showed a significant decrease in T-cell fractions as well as a relative increase in null cells, a normal distribution of B cells, a marked reduction in responsiveness to Con A, a lesser reduction to PHA and only a minor reduction to PWM. With increasing disease activity, the number of null cells increased despite lymphopenia. Spontaneous lymphocyte transformation was observed in patients with SLE. This occurred predominantly in the fractions enriched in B cells and was observed both early (0-16 hr) and late (68-72 hr) in the lymphocyte cultures. The method of discontinuous Ficoll gradients is both versatile and reproducible with good correlations between isolated lymphoid subpopulations and disease activity.