Anti-B-blocked ricin (anti-B4-bR) combines the specificity of the anti- B4 (CD19) monoclonal antibody with the protein toxin “blocked ricin.” In blocked ricin, affinity ligands are attached to the ricin B-chain to attenuate its lectin binding capacity. In a phase I trial, Anti-B4-bR was administered by 7-day continuous infusion to 12 patients in complete remission after autologous bone marrow transplantation (ABMT) for relapsed B-cell non-Hodgkin's lymphoma (NHL). Patients were treated at 20, 40, and 50 micrograms/kg/d for 7 days. Potentially therapeutic serum levels could be sustained for 3 to 4 days. The maximum tolerated dose was 40 micrograms/kg/d for 7 days (total 280 micrograms/kg). The dose-limiting toxicities were reversible grade IV thrombocytopenia and elevation of hepatic transaminases. Mild capillary leak syndrome was manifested by hypoalbuminemia, peripheral edema (4 patients), and dyspnea (1 patient). Anti-immunotoxin antibodies developed in 7 patients. Eleven patients remain in complete remission between 13 and 26 months post-ABMT (median 17 months). These results show that Anti-B4- bR can be administered with tolerable, reversible toxicities to patients with B-cell NHL in complete remission following ABMT.