Polyvinyl alcohol (PVA) mounting medium containing paraphenylenediamine (PPD), n-propyl gallate (NPG), or 1,4-diazobicyclo(2,2,2)-octane (DABCO) was compared with PVA alone or buffered glycerol with regard to capacity for preservation of immunofluorescence preparations. The results were based on staining of an artificial substrate with homogeneous antigen distribution followed by microphotometric determination of the initial light emission from bound fluorescein isothiocyanate (FITC)-labeled antibody and the subsequent fluorescence fading during 3-min exposure to blue excitation light. At a concentration of 0.2-2.0 g/liter and 6 g/liter, respectively, PPD and NPG were shown to effectively retard fluorescence fading without notably decreasing the initial emission intensity; two requisites were that the modified PVA used must be rather fresh and that the mounted preparations be examined within a few days. Although addition of DABCO (6 g/liter) afforded a mounting medium that tolerated storage before use better, but both PPD and NPG were more advantageous in practice. The retarding effect of PPD on fading of FITC emission was confirmed by performance testing on human tissue sections. Remounting in PVA alone is recommended for prolonged storage of sections that have been mounted in PVA modified with one of the above-mentioned compounds.