The lysis of allogeneic cells by cytolytically active T lymphocytes has been shown to involve two stages, Each with distinct cation requirements. Cytolysis required the presence of Ca++ ions, but addition of Mg++ to Ca++ containing medium synergistically enhanced target cell destruction. This synergistic effect resulted form kinetically separable events mediated by these cations. Thus, optimal adhesions between killer and target cells necessitated the presence of Mg++; interactions also occurred, but less efficiently, in the presence of Ca++. Lymphocyte-target cell interactions resulting from the present of Mg++ did not lead to the lysis of the target cell, until Ca++ were added. A "late" stage of the establishment of a target cell lesion is thus uniquely served by Ca++.