Analysis of viable and nonviable cells

Provided are methods, for selectively analyzing a cell sample. Viable cells are dyed with a membrane-permeable fluorescent dye, and nonviable cells are dyed with a membrane-impermeable fluorescent quenching dye. The cells are illuminated to cause fluorescent emission from the membrane-permeable fluorescent dye in the viable cells and the membrane-impermeable fluorescent quenching dye in the non-viable cells. The cells are then quenched for at least a portion of fluorescence of the membrane-permeable fluorescent dye in the nonviable cells by the membrane-impermeable fluorescent quenching dye. The cells are then analyzed for viable and nonviable cells.