Anti-NALL-1 rabbit IgG was incubated with NCS, an unique membrane-reactive anticancer antibiotic, in the presence of water-soluble carbodiimide. The resulting mixture contained NCS-immune IgG. The conjugates inhibited the growth and (3)H-TdR incorporation of human leukemia cell line (NALL-1) cells. A membrane immunofluorescent test with FITC-labeled rabbit anti-NCS and goat anti-rabbit IgG antibodies showed specific localization of NCS-immune IgG on NALL-1 cell surfaces. These results indicate that NCS-immune IgG retains both NCS and antibody activities, and NCS-immune IgG should be useful for cancer therapy.