51Cr-release assay has been used to measure cell-mediated cytotoxicity. However, this method requires an RI facility and the half-life of 51Cr is only 28 days, making it unsuitable for routine application. We therefore labeled target cells with carboxyfluorescein diacetate (cFDA), convertionally used to determine cell survival, to measure cell-mediated cytotoxicity by flow cytometry, and obtained the following results. 1. cFDA did not adversely affect the proliferation of K562, Daudi and Raji

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... lls. cFDAlabeled cells also proliferated well. 2. The most stable histograms of flow cytometry were obtained under the conditions of 150 ug/m/ cFDA concentration and 30 minutes of labeling time. 3. A high correlation was found between this method and "Cr-release assay. 4. The above results suggested that this method was very useful for measuring cellmediated cytotoxicity.