Lymphocyte-mediated lysis of target cells grown as monolayers in microtiter wells is readily quantitated by an assay measuring the 86Rb incorporated after attaining isotopic equilibrium. Lysis of fibroblasts by allogeneic lymphocytes sensitized by skin grafts and of tumor cells by syngeneic spleen cells sensitized by intraperitoneal tumor inoculation were readily detected. Weakly cytolytic lymphocyte populations can be assayed by increasing incubation times to 48 h or longer. A potential problem, 86Rb incorporation by lymphocytes sticking to residual target cells, was controlled by comparing 86Rb incorporation by targets incubated with non-immune lymphocytes. Results by 86Rb incorporation were identical to those determined by microscopic counting or 51Cr release. 86Rb incorporation assays should be considered as an alternate to 51Cr release techniques, especially in those experimental systems where the cytolytic potential of a lymphocyte population is so low that lysis can be detected only after long incubation times and/or when the spontaneous release of 51Cr is prohibitively high.