Components of the immunosuppressive regimen used to reactivate latent Pneumocystis carinii infection were analyzed for their effects on the growth, nutrition, and lymphoid system of hosts. Rats that were administered either tetracycline or a low-protein (8%) diet alone for 7 weeks developed few abnormalities, but animals on the combined regimen developed lower body and lymphoid organ weights, lower serum albumin levels, and fewer circulating lymphocytes. Rats that were administered corticosteroids and tetracycline experienced severe wasting, debilitation, and generalized lymphocyte depletion; the low-protein diet increased the magnitude of these changes. Alterations in the frequency of occurrence of specific lymphocyte subsets occurred only in rats given corticosteroids and consisted mainly of a greater decline in peripheral blood T helper cells than in T suppressor cells. The data suggest that long-term tetracycline administration and a low-protein diet have a variety of adverse effects on the host which enhance the immunosuppressive properties of corticosteroids.