Immunotoxicity assay that can be used to assess either natural killer (NK) cell or cytotoxic T cell activity or macrophage activity. When incubated with \(\ce{^{51}Cr{-}labeled}\) chicken red blood cells (cRBCs), NK and cytotoxic T cells lyse the cRBC and thus induce the release of \(\ce{^{51}Cr}\) into the medium. In contrast, macrophages phagocytose cRBC, and are then lysed to assess the amount of \(\ce{^{51}Cr}\) they have taken up.