The lymphocytes infiltrating the bladder mucosa of 28 patients treated with bacillus Calmette-Guérin (BCG) for superficial bladder carcinoma were characterized using an immunohistochemical technique on frozen sections of biopsy specimens obtained during cystoscopy. The inflammatory response induced by BCG consisted mainly of T lymphocytes (CD3+), most of which had the helper/inducer phenotype (CD4+), with a CD4/CD8 ratio greater than 1. A minor subset of lymphocytes were of B phenotype (CD22+). These findings persisted for the whole follow-up period (6-12 months) in spite of a progressive decrease of the inflammatory infiltrate. No difference in the lymphocyte phenotype was observed between nonresponding patients and those who responded to BCG in the short term. It is concluded that, although intravesical BCG therapy does affect the immunocompetent cells of the bladder wall, the BCG-induced antitumor activity is unlikely to depend exclusively on a local immune mechanism.