Cellular immunologic reactions occurring in the bladder after intravesical treatment with bacillus Calmette-GuΓ©rin (BCG) were investigated by flow cytofluorometric analysis of leukocytes present in the urine. Urine specimens from 11 superficial bladder cancer patients were collected before and 5, 24, 48 and 72 h after repeated BCG instillations. Monoclonal antibodies specific for granulocytes, monocytes/macrophages, and T-and B-lymphocytes were used to characterize and quantify leukocyte subpopulations. The total number of cells in urine was found to be 2- to 485-fold increased 24 h after BCG administration. The predominant cell type present was the polymorphonuclear granulocyte, probably representing a defense mechanism against mycobacteria. The main mononuclear leukocytes in urine specimens were monocytes/macrophages and T-lymphocytes, indicating an ongoing immune response in the bladder wall. Although percentages of lymphocytes were low, T- and B-cells could be identified using a selective cell measurement procedure. In conclusion, a clear increase in the numbers of granulocytes, monocytes/macrophages and T-lymphocytes in urine after intravesical BCG administration was demonstrated, indicating local activation of the immune system.