Monoclonal antibody to L3T4 has been used successfully to suppress autoimmunity in the New Zealand black/New Zealand white F1 (B/W) mouse model for systemic lupus erythematosus. To clarify the immunopathology of murine lupus and determine the effects of anti-L3T4 treatment on the cellular composition and histopathology of lymphoid organs, we examined the distribution of lymphocyte subsets in cryostat sections of the thymus, spleen, and lymph nodes of B/W mice. Immunohistologic specimens were obtained from female B/W mice that had received weekly intraperitoneal injections of either rat monoclonal antibody to L3T4 (2 mg/mouse/week) or phosphate buffered saline (200 microliters/mouse/week) from age 5 months until euthanasia at 8 months. B and T cell domains in each organ were identified on serial sections with monoclonal antibody directed against B220 (all B cells), Thy-1.2 (all T cells), L3T4 (helper T cells), and Ly-2 (cytotoxic/suppressor T cells). In control mice, striking cytoarchitectural abnormalities were identified in the thymuses, and the spleen and lymph nodes were hypertrophied relative to anti-L3T4 treated mice. Thymic abnormalities included amplification of medulla, formation of thymomas, and cortical atrophy. Amplified medullary regions and thymomas in B/W mice contained numerous B cells and L3T4+ T cells but few Ly-2+ T cells. The enlarged spleens and lymph nodes of control mice consisted of numerous secondary follicles with germinal centers containing an unusual subpopulation of T cells that expressed L3T4 but not Thy-1.2. In contrast, mice treated with anti-L3T4 did not develop histopathologic changes characteristic of systemic lupus erythematosus in any organ. However, treatment depleted L3T4+ cells from the spleen and lymph nodes, and it modulated the expression of L3T4 by thymocytes. These observations demonstrate that treatment with anti-L3T4 not only interferes with L3T4-dependent T cell functions, but it also prevents progressive abnormalities in lymphoid tissue in lupus-prone B/W mice. This preservation of normal lymphoid structure may contribute to the beneficial effects of anti-L3T4 on autoimmunity.