A study was undertaken to obtain baseline information for future studies of H-antigen expression in urothelial tumors. The influence of ABO- and Lewis genes on H-antigen expression in human ureters was investigated. Thirty two ureters of different ABO- and Lewis types were stained by the Tween 20-modified immunoperoxidase staining technique using Ulex europaeus agglutinin I (32 cases) and two monoclonal anti-H antibodies (10 cases). Ureters from A and AB individuals were also stained using monoclonal anti-A and Dolichos biflorus agglutinin (14 cases). The H-antigen expression in endothelium of all cases and in urothelium of Lea-b+ and Lea-b- individuals was correlated to ABO blood type. H antigen was never detected in urothelium of Lea+b- individuals. Endothelial H-antigen expression was not influenced by the Lewis types. In A and AB individuals, urothelial and endothelial H- and A-antigen and Dolichos biflorus reactivities were mainly insignificantly related, with correlation coefficients ranging numerically from 0.10 to 0.72. The study stresses the importance of relating blood group antigen H expression in urothelial tumors to the ABO-, Lewis, and secretors types of the individual.