Blood group antigens have been implicated as markers in oncogenesis; however, studies of blood group antigen expression in the prostate have yielded conflicting data. This study examined the expression of A, B, H, X, and Lewis antigens in 30 prostates removed for adenocarcinoma, by utilizing monoclonal antibodies, Ulex europaeus agglutinin I (for H type-2 chains), and the secretor status. A, B, and Ulex staining of the normal prostate most commonly demonstrated strong straining of 5-15% of cells. Staining of central and peripheral zones was similar, and nodules of hyperplasia showed no difference in staining. Antibodies to Le(a), Le(b), and X showed no staining or only pale staining of less than 10% of the normal prostatic epithelial cells. Ulex staining of intermediate and high-grade areas of tumor showed a diffuse staining pattern that was consistently greater than that of the normal glands. In contrast, low-grade tumors stained similarly to normal glands with Ulex. In 15 of 16 cases of dysplasia, Ulex staining was increased over normal and was similar to the adjacent carcinoma. A, B, Le(a), Le(b), and X antigens were negative in adenocarcinoma and dysplasia. This study supports the hypothesis that dysplasia represents a premalignant lesion; it further suggests that concomitant with malignant transformation, a disturbance occurs in enzyme activity. The result is augmentation of H type-2 expression in dysplasia and less-differentiated adenocarcinomas, and a loss of A, B, Le(a), and Leb expression in dysplasia and all carcinomas.