a method of staining living cells with special stains used in nontoxic concentrations. The dyes may be basic, such as neutral red and methylene blue (a chromophore group bonded to a cation), or acidic, such as phenol red and cyanol (a chromophore group bonded to an anion).

Upon penetrating an animal cell, some stains may diffusely stain the cytoplasm, others are deposited in the form of granules in the region of the Golgi complex, leaving the nucleus and cytoplasm unstained. When the cells are damaged, staining with diffuse stains is intensified, whereas granular ones lose the ability to form granules and stain the cytoplasm and nucleus diffusely. In living plant cells the stains condense in the vacuoles; in dead cells they stain the entire protoplast. These characteristics make it possible to differentiate dead and damaged cells from living ones.

Calculation of the amount of stain bound by cells provides a means for evaluating more subtle shifts in their functional state. Vital cytophotometry is used to determine the amount of dye bound in a specific cell and even in different parts of the cell. Acidic granular dyes are used to expose elements of the reticuloendothelial system and to study their condition. Methylene blue is used for selective staining of individual neurons. Some stains serve as indicators of hydrogen-ion concentration and oxidation-reduction potential. The distribution of stains of the fluoro-chrome group within the cells is studied by means of a fluorescence microscope. These stains serve to evaluate the viability of cells and are used in certain cytochemical studies.