Abstract
Comparisons of the cytotoxic activity of mononuclear cells from venous blood of human subjects against human melanoma cell lines have been made by means of the ^51^CV release assay (CRA) and the microcytotoxicity assay (MCA) of Takasugi and Klein. The relative effectiveness of the two assays in the detection of killing by various subβpopulations of effector cells was studied after depletion of monocytes and separation of the blood monocluear cells by SRBC receptor and Fc receptor rosetting techniques. Overall, the results showed good correlation. Most differences were the result of low values recorded with the CRA when effector cells were obtained from HypaqueβFicollβseparated populations which had not been further fractionated. These differences were largely removed when monocytes were depleted by glass absorption and may reflect interference with the CRA by monocytes. No consistent differences were found between the assays in the detection of killing by subfractions enriched for T cells or B cells in this tumour system. The MCA appears to offer the advantage of greater sensitivity and possibly detection of a wider range of cytotoxic mechanisms but the CRA has a number of advantages of a technical nature which make it more applicable to routine assays in clinical studies.