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A new in vitro microassay was devised for the quantitation of macrophage-mediated cytotoxicity on cancer cells. After 72 h of culture with macrophages, the residual adherent target cells were stained with methylene blue. The cell-bound dye was then eluted and measured in a photometer. This assay is simple, quickly performed and gives reproducible results in good correlation with direct counting of the residual cells. It allows a quantitative evaluation of the whole toxic effect (cytostasis and cytolysis) of activated macrophages on adherent tumour cells.