Lymphocyte subpopulations (B cells, CD4, CD8), interleukin-20 receptors (IL-2), monocytes/macrophages (Leu M5), and HLA-DR antigen expression were studied immunohistochemically on frozen sections from 38 bladder cancer specimens. T cells predominated over B cells in all tumours. CD4-positive lymphocytes predominated over CD8 in the stroma (CD4/CD8: 1.35/l), while in epithelial tumour cells CD8 was the prominent subpopulation (CD8/CD4: 1.75/l). Aberrant HLA-DR expression was found in 21.05 per cent of bladder tumours. A strong correlation between CD4 and CD8 population densities and macrophages with the other subpopulations was noticed. In HLA-DR-positive tumours, there was no correlation of the percentage of positive cells with CD4- and CD8-positive lymphocyte populations. Various parameters including IL-2 receptors, B cells, CD8- and CD4-positive cells, and macrophages did not differ significantly between the groups of tumours expressing and not expressing HLA-DR antigen. There were no statistically significant differences in the population densities of B cells, CD8- or CD4-positive cells, IL-2 receptor, monocytes/macrophages, and HLA-DR antigen expression among various clinicopathological parameters, including growth pattern, histological grade and clinical stage or patient's age and sex. These findings suggest that in transitional cell carcinoma of the urinary bladder, HLA-DR antigen expression is independent of lymphocyte subpopulations. It is therefore possible that HLA-DR expression by tumour cells reflect the existence of separate HLA-DR-positive or HLA-DR-negative tumour clones.