HLA class I and II glycoproteins from transitional cell carcinoma (TCC) and from perineoplastic and healthy vesical mucosa were characterized together with infiltrating cells by means of immunochemistry using specific monoclonal antibodies on frozen sections obtained during resection or radical cystectomy. Specimens were taken from 11 patients with TCC and five with healthy bladder mucosa. Four patients with TCC and four with healthy mucosa had been previously treated with a course of intravesical bacillus Calmette-Guerin (BCG). Ten out of 11 TCC samples expressed class I glycoproteins with a membrane pattern (diffuse in seven, focal in three) as normal epithelial cells from either controls or perineoplastic bladder. Interestingly, eight out of 11 TCC samples expressed class II antigens on their membrane that were also present in six cases in the perineoplastic tissue while the epithelial cells from four out of five normal bladders were completely negative. The epithelial display of class II antigens in the non-neoplastic areas and in the normal bladder correlates (p less than 0.001) with the degree of cellular infiltrate while such a relationship was not found between the HLA II expression of neoplastic cells and the infiltrate. BCG treatment was associated with a higher amount of inflammatory cells, prevalently T "activated" cells (CD5+,DR+), with a CD4/CD8 ratio always greater than 1. In the light of the role played by HLA glycoproteins in immune mechanisms, these results could help explain the positive action of BCG and the relative immunosensitivity of TCC.