The effects of long-term treatment with azathioprine and prednisolone on T- and B-lymphocytes were studied in 52 patients with lupus nephritis (LN) and chronic glomerulonephritis (GN). The effect of azathioprine on lymphocyte populations was dose dependent; high doses decreased the number of T- and particularly, B-cells, while smaller doses produced a selective depletion of B-cells. The changes in T- and B-cells during prednisolone treatment were variable with alternating increases and decreases in their numbers. The patients with increased B-lymphocyte levels showed the best response to immunosuppressive therapy.