Streptavidin-tricolor is a reliable marker for nonviable cells subjected to permeabilization or fixation.

Cell death is normally accompanied by loss of integrity of the cell membrane. Concomitant loss of osmotic pressure and permeability for DNA binding dyes like propidium iodide make it possible to distinguish viable from nonviable cells. However, after permeabilization for intracellular staining or after fixation of the cells, we find that propidium iodide leaks out of nonviable cells and is transferred to formerly viable cells. Cell size cannot be used for examining viability in permeabilized or heterogeneous cell populations. Here we show that streptavidin-tricolor enters specifically and irreversibly into dead cells, and is not transferred to formerly viable cells after fixation or permeabilization. Therefore, streptavidin-tricolor can be a useful dead-cell marker in experimental situations where conventional methods fail to distinguish between viable and nonviable cells.