The in vitro reaction between sensitized lymphoid cells and target cells has been studied in an allogeneic transplantation system in mice. Mastocytoma cells of the DBA/2 donor strain were injected into C57BL mice, and the spleens of the recipients harvested 8 days later. The immune lymphoid cells thus obtained were tested for their ability to damage in vitro cultures of target mastocytoma cells. The reaction was followed by two methods: (1) microscopic counts, and (2) ability of target cells to form colonies in a semi-solid medium after various periods of contact with immune lymphoid cells. The results show: (a) that the effect on target cells is an exponential function of the number of immune lymphoid cells employed, and (b) that the reaction becomes noticeable after 3 hours and reaches completion within 12 hours when evaluated by cloning techniques, while it takes 24–48 hours to be microscopically clearly demonstrable. Problems arising from use of both methods are discussed.