An in vitro procedure for studying cell-mediated immunity in the mouse is described in which one can obtain cells (cytotoxic lymphocytes or CL) capable of lysing foreign (target) cells. The lysis is specific to antigens present during culture and is directed primarily against H-2 antigen differences although it is shown that non-H-2 antigens can also contribute. The relative number of CL produced in the cultures is also measured in vitro by a 51Cr release technique. By measuring the cytotoxicity produced by cells from each culture at several multiplicities of sensitized cells to target cells, it is shown that appropriate analysis can yield the actual ratio of CL in different cultures rather than the more qualitative measure obtained by merely measuring percentage of chromium release at fixed multiplicity. The procedure is extended to enable one also to quantitate relative numbers of progenitors of CL.