Abstract: The generation and activity of serum neuralizing antibody in cynomolgus monkeys and SD rats undergoing long-term toxicity study with antitumor peptides HM-3 were investigated.The rats were administered intravenously with HM-3 at doses of 4.5 mg/kg，13.5 mg/kg，and 40.5 mg/kg for 6 months，and the cynomolgus monkeys were administrated intravenously with HM-3 at doses of 3 mg/kg，9 mg/kg and 27 mg/kg for 6 months，respectively.Anti-HM-3 antibodies and their neutralizing activities in serum samples taken every month after the administrations were determined by ELISA and cell migration assay，respectively.During the long-term administrations，anti-HM-3 antibodies were generated in some SD rats at high and moderate dose groups，and the anti-body-neutralizing activities could be detected in a number of these samples (P<0.05).However，activity could be detected in very few monkeys (P<0.05)，and the antibody titers were not correlated with the neutralizing activities.Therefore，we conclude that the antigenicity of HM-3 was low，but after long-term administration at high dose，low affinity neutralizing antibody could be generated in small number of samples.