Six patients with progressive B cell non-Hodgkin's lymphoma have been treated with an IgG2a mouse monoclonal antibody (mAb) against the B cell differentiation antigen CD19, with total doses varying from 225 mg to 1000 mg. Free mAb was detected in the serum after doses of 15-30 mg. After the mAb infusions the number of circulating tumour cells was temporarily reduced, but in some cases antibody-coated cells remained in the circulation for several days. mAb penetrated to extravascular tumour sites; in general higher doses were required to saturate cells in the lymph nodes than to sensitize tumour cells in the bone marrow. mAb doses of up to 250 mg were given i.v. over 4 h without major toxicity. One patient twice achieved a partial remission after two periods of mAb treatment with an 8-month interval; the second remission lasted for 9 months. One patient showed a minor response. None of the patients made antibodies against the mouse immunoglobulin. Serum immunoglobulin levels were followed as a measure of the function of the normal B cell compartment; no significant changes were seen up to 6 months after mAb treatment.