A tissue culture line of a human malignant melanoma, SEKI, induced cachexia in nude mice (BALB/c-nu/nu). The melanoma produced a protein immunologically identical to human .alpha.1-antichymotrypsin (.alpha.1-Ach). Tissues of the SEKI melanoma contained the protein immunologically equivalent to 0.29 .+-. 0.11 (SD) mg of human .alpha.1-Ach per g wet tissue whereas the other 6 human malignant tumors transplanted into nude mice did not contain a detectable amount of it. In the serum of nude mice bearing the melanoma, this protein appeared soon after the tumor growth occurred and gradually increased up to the level equivalent to 5 mg of human .alpha.1-Ach/dl. Removal of the tumor resulted in a rapid decrease of the protein in the serum to an undetectable level within 1 day. This protein was never detected in the serum of nude mice bearing the other 27 human malignant tumors or in controls. Purification of this protein was carried out by column chromatography using DE-52, Blue-Sepharose and SP-Sephadex. The elution patterns were the same as those of .alpha.1-Ach in human serum, and the MW of the protein was estimated as 69,000 by Sephadex G-100 column chromatography and 65,000 by polyacrylamide gel electrophoresis with sodium dodecyl sulfate. This purified protein did not exhibit inhibitory activity against chymotrypsin. This melanoma produced a protein immunologically identical and physicochemically very similar to human .alpha.1-Ach. This melanoma-nude mouse system may provide a useful model for investigating the synthesis of human .alpha.1-Ach and analysis of its physiological roles.