The expression patterns of glycolipid from prostatic hyperplasia, prostatic cancer and normal prostate tissue were observed. A further analysis of antigen recognized by mouse monoclonal antibody APG1, which was gained by immunizing glycolipids extracted from human prostate cancer, was also performed. In cancer tissue, both of the lactosyl and globoside series glycolipids were found to be generally reduced, although in the ganglioside series, GM3 and GD3 were not reduced and only the glycolipids with longer chains than GD2 were found to be reduced. These results indicated that the inhibition of sugar chain elongation, but not sialylation, was the main synthetic change occurring with carcinogenesis of the human prostate. APG1 reacted with only two bands near GM2 and GD2 of the ganglioside fraction on a thin-layer chromatography plate, but it did not react with any of the known gangliosides of the ganglioside series including GM2 and GD2. Histochemically, APG1 showed intense reaction only in frozen tissue sections of human prostate, and the reactivity decreased with the increasing grade of cancer. Therefore, this antigen was considered to be a prostate-specific and differentiated antigen reacting with nonganglioseries gangliosides.