WITH the fluorescent antibody technique it is often difficult to see results; it is especially difficult to prepare good photomicrographs of positive structures with low fluorescence intensity. Short-term illumination of sections with concentrated ultra-violet and blue light causes the fluorescence intensity of fluorescein isothiocyanate (FITC) to decrease rapidly and dark, “burnt-out” areas are produced. To overcome this difficulty, a stabilization of FITC conjugates in microscopic slides was attempted by inclusion in the embedding medium of substances containing −NH2 groups such as dinitrophenylamine and tris-trihydroxymethylaminomethane, or by increasing the pH of the medium.