Abstract: An agent for enhancing antitumor activity of antitumor agents comprising as the active ingredient diltiazem or a pharmaceutically acceptable acid addition salt thereof, an antitumor composition comprising the diltiazem or an acid addition salt thereof and a conventional antitumor agent, and further a method for treating tumors by administering diltiazem or an acid addition salt thereof together with a conventional antitumor agent in oral or parenteral route to patients suffering from various tumors. Said active diltiazem or a salt thereof is effective for enhancing the antitumor activity of antitumor agents because it can enhance sensitivity of tumor cells to antitumor agents regardless the tumor cells have resistance to the antitumor agents or not.

Abstract: An antibody of a human leukemia virus-related peptide obtained by collecting an antibody produced in a mammal body by administering to the mammal an antigen prepared by reacting a human leukemia virus-related peptide selected from the group consisting of:(a) a peptide represented by formula (1):H-Tyr-Val-Glu-Pro-Thr-Ala-Pro-Gln-Val-Leu-H (1)(b) a peptide represented by formula (2):R-Ile-Pro-His-Pro-Lys-Asn-Ser-Ile-Gly-Gly-Glu-Val-OH (2)wherein R is the same as drefined above;(c) a peptide represented by formula (3):R-Thr-Trp-Thr-Pro-Lys-Asp-Lys-Thr-Lys-Val-Leu-OH (3)wherein R is the same as defined above;(d) a peptide represented by formula (4):H-Val-Val-Gln-Pro-Lys-Lys-Pro-Pro-Pro-Tyr-OH (4)(e) a peptide represented by formula (5):R-Met-Gly-Gln-Ile-Phe-Ser-Arg-Ser-Ala-Ser-Pro-OH (5)wherein R is the same as defined above; and(f) a peptide represented by formula (6):H-Tyr-Pro-Glu-Gly-Thr-Pro-Lys-Asp-Pro-Ile-Leu-Arg-Ser-Leu-OH (6)as a hapten, with a carrier in the presence of a hapten-carrier binding agent.