Rabbit xenoantiserum was produced against a human null cell type leukemia cell line (NALL-1), and immune IgG was purified. NCS, a unique antitumor antibiotic that works on the tumor cell surface, was reacted with anti-NALL-1 rabbit IgG, in the presence of water soluble carbodiimide (WSCD). The resulting mixture was chromatographed on a Sephadex G-200 column. The first and second fractions were shown to contain NCS-immune IgG and no free NCS by the Ouchterlony double diffusion method and by immunoelectrophoresis. The conjugates inhibited the growth of Sarcina lutea and were more stable at conditions of 37℃ or 56℃ than free NCS. The conjugates did not loose inhibitory activities against the growth of S.lutea after frequent freezings and thawings. NCS-immune IgG might be useful in treating patients with cancer.