AbstractSome dyes used in histotechnology fade within months; others remain unaltered for many years. Problems of fading are well known since antiquity and have been thoroughly investigated. Yet textbooks of histotechnology do not provide information concerning the sensitivity of dyes to light, acids, alkalis, and other reagents, though quantitative data are easily accessible in technical and chemical publications. Investigations in this laboratory demonstrated the usefulness of these data for the design of new reactions and modifications of traditional technics to prevent or minimize fading. It seemed, therefore, of interest to review pertinent literature on fastness properties of dyes. This report will deal mainly with standards of the fastness of dyes to light, acids and alkalis, effects of different light sources, dye concentration, and fluorochromes. (The J Histotechno1 12:57, 1989.)