OBJECTIVES: I. Evaluate the efficacy of rituximab in patients with B-cell lymphoproliferative disorders while under pharmacologic immune suppression for control of either allograft rejection or autoimmune disease. II. Evaluate the safety and direct toxicity of rituximab in this patient population, including the potential for opportunistic infections. III. Evaluate the secondary consequences of rituximab therapy in this population, including changes in the requirement for immunosuppressive drugs, effects on graft rejection, graft survival, and severity of autoimmune disease.