A series of 76 bladder tumours was studied using a panel of 5 anti-human leucocyte monoclonal antibodies (mAb): anti-pan-leucocytes (SLC1), anti-T lymphocytes (ST1), anti-B lymphocytes (SB3), anti-macrophages (PHM2) and anti-granulocytes (WEMG1). The DNA content and the expression for each mAb were measured in separate samples with flow cytometry. The importance of local inflammatory reaction was measured objectively according to tumour grade. These cells formed 41, 32 and 31% of the total cell count for grade I, II and III tumours respectively. The percentage of each leucocyte population according to grade I, II or III was as follows: 8, 5 and 6% respectively for T-lymphocytes; 3, 3 and 4% for B-lymphocytes; 10, 7 and 6% for granulocytes; 17, 18 and 24% for macrophages. No relationship was found between mAb expression and DNA content of tumours. This study demonstrates the importance of the inflammatory reaction in bladder tumours and the preponderance of cells expressing a macrophage phenotype. In a flow cytometry study, leucocytes may mask the presence of a minor group of urothelial tumour cells with an elevated DNA content associated with a poor prognosis; on the other hand, host leucocytes within a tumour could be used as an internal reference for precise measurement of the DNA content of tumour cells.