Cytotoxicity assays provide an in vitro evaluation of the lytic activity of NK and T cells against tumors or transformed cells. However, none of these methods allow the recovery of cells or supernatants after the assay. We standardized a microcytotoxicity test using calcein-acetoxymethyl (calcein-AM) dye that requires very small quantities of cells while maintaining the same sensitivity as the traditional (51)Cr assay. The assay is applicable to resting as well as activated human effector cells and uses different targets such as human cell lines that are adherent or growing in suspension and resistant or sensitive. The most important feature of the method is the possibility of recovering cells and supernatants for additional analyses such as phenotyping and evaluation of soluble factors.