High-content screening (HCS) and other quantitative cellular imaging assay methods that use fluorescence microscopy require effective fluorescent labeling and identification of the target cell(s). Fluorescent probes that stain cells as the primary objects are used to identify and count individual cells, as well as to define the region(s) of each cell to which target-specific image analysis is applied. For this purpose, the primary object can be a major component of the cell, such as the nucleus or a large organelle, or the whole cell itself. When the whole cell is the primary object, a high-quality cell stain is needed to delineate the intact cell from bordering cells without also interfering in target-specific detection and analysis. Thermo Scientific Cellomics ® Whole Cell Stains (Thermo Fisher Scientific Inc., Rockford, Ill.), available in blue, green, orange and red, provides image staining and the ability to quantify the whole cell volume in HCS and fluorescence microscopy assays.