Rituximab is a chimaeric monoclonal antibody which specifically binds to the CD20 antigen on normal and malignant B lymphocytes. It produces antibody-dependent cell- and complement-mediated cytotoxicity in these cells. Rituximab reduced peripheral B lymphocyte counts by approximately 90% within 3 days in patients with relapsed indolent lymphoma. Counts remained depleted for 6 months and recovered by months 9 to 12 after 4 doses of rituximab 375 mg/m2 once weekly. Clinical response rates were 46 and 48% in 2 non-comparative trials in patients with relapsed indolent lymphoma. The rate of response to rituximab appeared to be markedly higher in patients with follicular lymphoma than in those with small lymphocytic disease (56 or 60% versus 13 or 15%). 85 to 94% of patients reported adverse events during clinical trials of rituximab; 90% of events were mild or moderate. The most common adverse event, a transient set of flu-like symptoms during the first infusion in approximately 50 to 87% of patients, generally resolved completely in < 3 hours. Diphenhydramine and/or paracetamol was administered to some patients. In 10% of patients, the flu-like symptoms during the first infusion were accompanied by bronchospasm and/or hypotension or severe cytokine release syndrome. Patients were generally able to complete treatment after these symptoms revolved.