Peritoneal macrophages labelled with 51Cr in suspension and cultivated for 48 hr on glass pretreated with poly-L-lysine were used as target cells for determination of the cytotoxic effect of immune lymphocytes. 51Cr release from such target cell from different strains of mice is 15.5 +/- 0.8% of the total target cell radioactivity after 20 hr incubation with normal allogeneic lymphocytes. The use of 2% sodium dodecylsulphate ensures 100 percent solubilisation of labelled target cells growing on the glass surface and permits the cytotoxic effect to be determined by both 51Cr release and measuring the label retained by the intact cells. The two methods proved to be accurate, reproducible and in accord with each other as well as with the method of direct cell counting. Determination of released 51Cr enables the cytotoxic effect of lymphocytes to be measured after 4 hr incubation with the macrophage monolayer. SaI and Mc11 ascitic sarcomas display different sensitivities to the cytotoxic effect of immune lymphocytes and require different optimum conditions for its development.