To monitor the distribution of lymphocyte sub-populations infiltrating tumors, a simplified modification of standard techniques was developed to identify the complement-receptor B lymphocytes in frozen tissue sections. This technique is based on binding of antibody-coated bovine erythrocytes by B lymphocytes in the presence of sub-hemolytic concentrations of mouse complement. B lymphocytes can be identified in frozen tissue sections by adherent sensitized bovine erythrocytes. The specificity of the technique for B lymphocytes was established by 1) absence of spontaneous rosette formation with bovine erythrocytes by human T lymphocytes, 2) specific binding of bovine erythrocyte-antibody-complement complexes to B cell regions of normal lymph nodes and spleen and 3) binding of erythrocyte-antibody-complement to a high proportion of chronic lymphocytic leukemia (a B cell leukemia) lymphocytes. Five transitional carcinomas studied had regions of mononuclear infiltration that were virtually devoid of complement-receptor lymphocytes. The results suggest that lymphocytes infiltrating bladder carcinomas may be predominantly T lymphocytes.