Cell surface antigens of normal urinary bladder epithelium (NBE) and bladder cancers of inbred ACI/N rats were assayed by antisera raised in rabbits. Mixed hemadsorption test combined with absorption analysis defined three kinds of cell surface antigens in both NBE and bladder cancer cells; the first antigen was common to lymphoid cells, the second was present in most of the epithelia, and the third was unique to NBE and present to a lesser extent in bladder cancers. In addition, bladder cancers had a fourth antigen that was common only to epidermis. These findings were confirmed by the use of typing sera for the third and the fourth antigens, which were prepared by absorptions of each antiserum with lymphoid cells and the lung. Direct titration of the typing sera against various cells revealed that the third antigen (NBE antigen) was species-specific and the fourth antigen (BC-SK antigen) was not common to human bladder cancers but common in part to mouse epidermal cells and bladder cancers.