By using autoradiographic methods, we have studied the actinomycin-binding in nuclei of CLL's lymphocytes cultured, in presence of PHA, at varying cellular concentrations. Chromatin activation is earlier and greater in the most concentrated culture, which thus more closely resemble normal lymphocytes' cultures. This data supports the hypothesis that T cells in CLL have normal PHA-stimulability and that their apparently delayed response is due to dilution in a predominant B cell population.