The immunophenotypic characteristics of lymphocyte subpopulations in tissue lesions of human tuberculosis were investigated in 18 cases. Frozen sections of affected cervical lymph nodes were stained with a panel of monoclonal antibodies using avidin-biotin immunofluorescence technique. The mean percentage of cells staining with different antibodies was as follows: CD3 44%; CD4 43%; CD8 36%; HLA-DR 86%; and IgM 29%. The mean CD4/CD8 ratio was 1.3:1. Leu7 and CD1 staining cells were very sparsely distributed in the granulomas. Thus, infection with tuberculosis is associated with a predominantly T cell infiltrate in lesional tissues. The proportion of CD4 positive (helper/inducer) cells at these sites is much lower than that seen (i) in the normal circulation, (ii) in reactive lymph nodes and (iii) at the site of purified protein derivative (PPD) injection in healthy tuberculin-responsive individuals.