An improved method and apparatus for staining samples of biological material for accurate analysis of the sample. Biological material is applied to a substrate, such as a microscope slide. The biological specimen is then stained with a selected staining composition, which may be gentian violet for a Gram's Stain analysis. The stained biological material is at least partially decolorized and the level of decolorization is analyzed optically. If necessary, the decolorizing step and the optical analysis steps are repeated until a selected level of decolorization is obtained.