OBJECTIVES: I. Compare the efficacy of CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) with or without rituximab in older patients with diffuse mixed, diffuse large, or immunoblastic large cell non-Hodgkin's lymphoma of B-cell lineage with respect to the response rate, time to treatment failure, toxicity, and survival. II. Compare the efficacy of maintenance therapy consisting of rituximab vs observation alone after initial therapy with respect to the time to treatment failure, duration of response, toxicity, and survival of this patient population. III. Determine if maintenance therapy with rituximab results in the conversion of any partial response to complete response.