Rituximab, a chimeric antibody that targets CD20(+) B cells, produces a 48% response rate in patients with refractory low-grade non-Hodgkin lymphoma. In this phase II trial, patients with low-grade non-Hodgkin lymphoma who had previously received no systemic therapy were treated with rituximab, 375 mg/m(2), administered by IV infusion for 4 consecutive weeks. Patients with objective response or stable disease received repeat 4-week courses of rituximab at 6-month intervals. At the time of initial reevaluation at 6 weeks, 21 of 39 patients (54%) had objective response to treatment, and an additional 14 patients (36%) had stable disease or minor response. Response rates were similar in patients with follicular and small lymphocytic (CLL-type) lymphoma (52% versus 57%, respectively). At present, follow-up is short and only 13 patients have undergone a second course of rituximab treatment. However, 4 additional responses were documented either prior to the second course of rituximab (2 patients) or following the second course (2 patients) and 4 patients improved from partial to complete response. The current response rate is 64%, with 6 complete responses (15%). Treatment with rituximab was well tolerated, with only 1 patient experiencing grade 3/4 infusion-related toxicity. Rituximab is well tolerated and highly active in patients with low-grade non-Hodgkin lymphoma previously untreated with systemic therapy. Although further follow-up is required, the demonstration of minimal toxicity and considerable activity of this new biologic agent represents an important beginning of more specific, less toxic treatment for this important group of cancer patients.