Direct estimation of frequency of cytotoxic T lymphocytes by a modified plaque assay

THYMUS-DERIVED lymphocytes (T) have been shown to have a major role in cell-mediated immunity against foreign transplants and syngeneic tumours ^1,2. In vitro, several assays have been developed to measure thymus-dependent cell-mediated immunity, making use of the cytotoxic potential of these effector cells³. Precise estimates of the number of cytotoxic T lymphocytes (CTL), however, are not possible by available methods. We report here a new method to quantify the CTL present in a mixed cell population. The method is based on the ability of CTL to kill specifically adjacent target cells fixed on monolayers and to form zones of lysis or plaques.