Summary— The lymphoid cellular infiltrates in the bladder biopsies from patients with carcinoma in situ before and after BCG intravesical therapy have been determined and characterised. This has been achieved using a panel of monoclonal antibody probes in an indirect immunoperoxidase technique. These studies have revealed the predominance of T-cells of the helper/inducer phenotype (T4+), β-lymphocytes (B1+) and macrophages (Leu M3+, 3.9+) in bladder biopsies after BCG therapy. HLA.DR antigens were also expressed on the lymphoid cells infiltrate as well as the urothelial cells. These results suggest that the components of an active immune response were present and enhanced in the bladder wall after BCG therapy.