Alteration of cell surface carbohydrate antigens during malignant transformation is a well-known phenomenon observed in various tumors. In prostatic carcinoma, nearly total deletion of normally occurring ABO and type I-based Lewis antigens, Le(a) and Leb, has been observed in several studies. We studied expression of the closely related type II antigens Le(x), Le(y), and sialyl-Lewis(x) (SLe(x)) using monoclonal antibodies. Thirty formalin-fixed specimens obtained from radical prostatectomy, containing prostatic carcinoma as well as benign tissue, were evaluated by immunohistochemistry. In both cancer and benign tissue, Le(x) expression was minimal or absent. In benign tissue, Le(y) was expressed in ducts and in the basal layer of glandular epithelium. In tumor tissue, Le(y) expression was greatly increased and extensive staining was observed in 26 of 30 cases. The SLe(x) expression in benign tissue was observed only in larger ducts, never in glandular secretory epithelial cells. In carcinoma, rare cells positive for SLe(x) were present in 8 of 30 cases, and stronger expression with focal to patchy distribution was observed in 14 of 30 cases. The results suggest an alteration in glycosyl transferase activity in prostatic carcinoma, with preserved or increased activity of enzymes responsible for the synthesis of the type II core sequence. This sequence is further glycosylated and expressed as the difucosylated compound Le(y) or the monofucosyl, monosialyl compound SLe(x). For prostate, Le(y) and SLe(x) are the only blood group-related antigens known to be minimal or absent in benign secretory epithelial cells that are more highly expressed in malignant tissue.(ABSTRACT TRUNCATED AT 250 WORDS)