The in vivo antitumor activity of NCS-immune immunoglobulin G (IgG) [Neocarzinostatin (NCS) conjugated with rabbit IgG antibody against a human leukemia cell line (NALL-1)] was evaluated in immunosuppressed newborn Syrian hamsters into which a transplantable human leukemia cell line, BALL-1 was implanted. After intraperitoneal (i.p.) injection of the conjugate, hamsters preinoculated i.p. with BALL-1 cells survived longer than hamsters treated with control solutions (p less than 0.01). The control solutions were NCS, immune IgG, a mixture of NCS and immune IgG, NCS-normal IgG conjugate and physiological saline. There was no change in body weight in the NCS-immune IgG-treated hamsters. The growth of subcutaneously (s.c.) implanted BALL-1 tumors was also inhibited by i.p. administration of NCS-immune IgG; however, the degree of inhibition was not significantly different from that obtained by administration of NCS alone, a mixture of NCS and immune IgG or NCS-normal IgG conjugate. These results indicate that NCS-immune IgG was effective against i.p. BALL-1 tumors, but was less effective against s.c. implanted tumors.