One of the so-called LC components, the content of which is increased in the serum of PSK (antitumor polysaccharide) treated mice, was purified by repeated ion-exchange column chromatography on DEAE-Sephadex A-50. The purified protein, designated as LC-2, was identified as a variant of mouse transferrin, which is a serum beta 1-globulin having an iron-binding capacity. The absorption spectrum and ultraviolet circular dichroism (CD) curve of LC-2 were identical with those of mouse transferrin. The molecular weight, isoelectric point and amino acid composition of LC-2 were in good agreement with the results obtained previously for mouse transferrin. However, the carbohydrate content of LC-2 was different from that of mouse transferrin. This protein restored the depressed spleen cell response of tumor-bearing mice to concanavalin A (Con A) and promoted the metabolism of proteose-peptone induced peritoneal macrophages obtained from tumor-bearing mice. Furthermore, it had weak but definite antitumor activity against Sarcoma 180 cells in vivo.