MRL-lpr/lpr mice develop massive lymphadenopathy with excessive proliferation of T cells and associated immune abnormalities. To examine whether to not the disease process is intrinsic and irreversible, an immunomodulatory drug, cyclophosphamide, was administered to 16-week-old, sick MRL-lpr/lpr mice. Analysis of lymph node cells by flow cytometry from injected and control MRL-lpr/lpr mice indicated that cyclophosphamide had a marked effect upon the lymphoid cellular composition. Whereas control lymph nodes had a very large number of abnormal dull Ly 1+ T cells and very few other cells, the cyclophosphamide-injected mice had normal T and B cells. The immune responses to a T-cell-dependent antigen, sheep red blood cells, and a T-cell mitogen, concanavalin A, were normalized in cyclophosphamide-injected mice as compared to controls. In addition, injected mice had prolonged survival, decreased arthritis, markedly reduced adenopathy and splenomegaly, improved renal histology, and significantly diminished autoantibody levels. This study suggests that the disease and associated immune abnormalities of MRL-lpr/lpr mice are reversible by selective elimination of the abnormal dull Ly 1+ T cells.