The ViaStain™ AOPI Staining Solution in PBS enables the user to quantitatively distinguish live and dead nucleated cells from a variety of primary mammalian cell samples, even in the presence of a high background of non-lysed red blood cells, platelets and/or debris using the Cellometer, Cellaca, and Celigo platforms. This formulation can work with whole blood, peripheral blood mononuclear cells (PBMC), bone marrow, splenocytes, thymocytes, lymph nodes, and hepatocytes but also works in various other digested tissues and cultured cell lines.