Summary: Small lymphocytes from lymph nodes of guinea pigs with tuberculin hypersensitivity react specifically with tuberculoproteins, as revealed by fluoresceinated guinea pig antituberculoprotein serum. Fluorescence occurs as a rimming or halo effect in 5 to 20% of such cells. Occasional glowing cells are seen in normal cell preparations, at a lower level of intensity of fluorescence. Although the methods employed suggest that antibody globulin adsorbed to cells, or produced by them, is not implicated in the reactive property, this possibility cannot be excluded.