Neocarzinostain (NCS) was first used by Hiraki and his colleagues for induction chemotherapy in acute leukemia. This new anti-tumor agent is a polypeptide with a high molecular weight of 10,700 daltons. Anti-NCS antibody was produced in rabbits administered NCS intramuscularly with or without adjuvant. The production of anti-NCS antibody in patients treated with NCS was investigated. Forty three leukemia cases of various types were examined totally 65 times. Two mg of NCS for four consecutive days by intravenous drip infusion followed by 7 to 10 days of pause was repeatedly administered. The total amounts ranged 8 to 174 mg and the total periods 4 to 87 days. The methods used to measure the antibody titer are the passive hemagglutination (PHA) test on microplate and the passive cutaneous anaphylaxis (PCA) reaction in guinea pigs. The sera of all patients showed only non-specific agglutination at less than 2(3) dilution by PHA test, and to confirm these results four patient sera were tested by PCA reaction. The production of anti-NCS antibody was not detected in patients by PHA test and PCA reaction. The anaphylactic reaction and other adverse reactions due to anti-NCS adtibody production were not demonstrated in patients. Anti-NCS antibody was not detected by these experiments in the dose schedule administered.