Influence of light and mounting medium on the fading of Feulgen stain.

Serial microspectrophotometric estimations of the absorbance of Feulgen-stained interphase nuclei of human fibroblasts were made on slides mounted in a variety of mounting media and stored in light or in darkness. All preparations showed some fading, at rates which varied with different mountants, were greater in the light than in the dark, and were greater in the monochromatic illumination of the microspectrophotometer under working conditions than in normal laboratory light conditions. Fading was minimised by using XAM improved white neutral mounting medium (G.T. Gurr), storing slides in darkenss, and by reading samples as quickly as possible.