Lym-I is a murine IgG2a monoclonal antibody (MAb) that is B-cell specific but has greater avidity for malignant B cells when compared with normal B lymphocytes. It was originally produced by immunizing mice with nuclei of cultured cells from a patient with Burkitt's lymphoma. Ten patients with progressive refractory B-cell malignancies were treated with 131I-labelled Lym-I. Treatment with 131I Lym-I produced complete or partial remissions in 4 patients. Toxicity did not occur or was mild in most patients. The only significant complications included two instances of fistula secondary to necrotic lymphoma and one instance of hypotension. Human antimouse antibodies occurred in only 2 patients after multiple injections of Lym-I antibody. This experience was in contrast to treatment of B-cell malignancies with unconjugated Lym-I alone. Unconjugated Lym-I also caused no significant toxicity but was less effective than 131I Lym-I.