Monoclonal antibodies were used to examine the pattern of distribution of lymphocyte subpopulations and macrophages in normal human urothelium. T lymphocytes (T11-positive cells) were demonstrated within the epithelium and within the lamina propria of both the ureter and the urinary bladder. Suppressor/cytotoxic T cells (T8-positive cells) predominated, with the average ratio of suppressor/cytotoxic T cells to the helper/inducer T cells being approximately 8.9 in the epithelium and 1.5 in the lamina propria. Leu M3+ cells (monocytes/macrophages) were detected in both epithelium and lamina propria. The character and distribution of these immunocompetent cells within the urothelium suggest a protective function, especially against infection and in tumour surveillance.