Studies at the NIH Clinical Center have shown that intermittent pulse cyclophosphamide therapy is effective for treating patients with severe lupus nephritis, but may result in substantial rates of sustained amenorrhea. Initial studies in patients with autoimmune rheumatic diseases have also suggested a beneficial effect from the lymphocyte-specific nucleoside analogs chlorodeoxyadenosine and fludarabine. Cyclophosphamide induces DNA cross-links whereas, nucleoside analogs inhibits DNA repair indicating complementary and partially synergistic modes of action. Whether combination of lower doses of cyclophosphamide with nucleoside analogs will increase efficacy while at the same time minimize toxicity from higher-cumulative doses of cyclophosphamide has not been determined. In this phase I/II study, 15 patients with proliferative lupus nephritis will be treated as outpatients with a combination of oral cyclophosphamide (500 mg/m(2)) on day 1 followed by fludarabine (30 mg/m(2)) subcutaneously on days 1, 2 and 3 every month for 3 cycles. The cumulative dose of cyclophosphamide in this regimen is approximately 2.5g as compared to greater than or equal to 30g in the standard NIH cyclophosphamide regimen. In this study the tolerance and toxicity of this combination will be studied. Regeneration of T and B cells following depletion including analysis of antigen-repertoire and function will also be examined. Preliminary efficacy information, including rates and time to renal remission and rates of preservation of renal function, will be analyzed to be used for future controlled studies. Pharmacokinetic analysis will be performed on a subset of patients to determine the bioavailability and pharmacokinetic parameters of subcutaneous fludarabine.