Human peripheral blood lymphocytes are stained with the metachromatic dye acridine orange and the fluorescence of individual cells is measured by flow cytometry. The relative content of stainable RNA per cell is estimated by comparison with RNase-treated cells. Non-T and T lymphocytes have different mean quantities of RNA per cell, and these classes exhibit different distributions of RNA content. Non-T cells have a unimodal distribution with a sharp peak and exponential distribution towards higher RNA values. T cells have a bimodal distribution with two separate peaks. When T cells having receptors for IgG (Tgamma cells) and IgM (Tmu cells) are separated, each of these cell populations displays a unimodal distribution. Of these three lymphocyte subpopulations, Tgamma cells have the lowest content of RNA per cell. Non-T cells have slightly higher RNA content than Tgamma, and Tmu cells have twice as much RNA as Tgamma cells. The RNA content, which surely relates to the different functions of these lymphocyte subpopulations, may also be a useful marker for rapidly distinguishing the lymphocyte subpopulations.