A phenomenon we have termed spontaneous lymphocyte-mediated cytotoxicity (SLMC) by non-thymus-derived lymphocytes from normal donors has been described. The phenomenon can be demonstrated using human and xenogeneic (mouse) cell lines as the target cell in a microplate 51Cr release assay which is simple and reproducible. In this paper, SLMC against xenogeneic targets has been evaluated as a potential marker of lumphocytotoxic function with respect to: (a) the nature of the target cell; (b) the variability of the cytotoxic function of lymphocytes from different donors, and from the same donor tested on different days; (c) the nature of the effector cell. Using buoyant density centrifugation of iron plus magnetpurified lymphocytes forming rosettes with sheep red blood cells (SRBC) (T-cells) or SRBC-rabbit 19S anti-SRBC-mouse complement (complement receptor lymphocyte), it has been demonstrated that the cytotoxic activity lies in the non-T-lymphocyte fraction, and is probably caused by the complement receptor-bearing lymphocyte. The potential usefulness of this phenomenon as a functional marker of non-T-LYMPHOCYTE CYTOTOXIC Ability, and for the assessment of serological factors ehich may affect this cytotoxicity, has been discussed.