Bleaching of stained objects is a major problem in immunofluorescence. The prevention of fluorescence fading would allow longer observation times, photographic documentation, fluorometry, and pattern recognition. Fluorescein kinetics and fluorescence intensities (FI) of fluorescein isothiocyanate (FITC) conjugate-stained Sephadex beads were studied with previously described "antibleaching" reagents using an argon laser as the excitation light source. Eight antibleaching reagents were tested (sodium azide (NaN3), sodium iodide (NaI), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), 1,4-di-azobicyclo-(2,2,2)-octane (DABCO), p-phenylenediamine (PPD), n-propylgallate, and sodium dithionite (Na2S2O4]. Sodium azide and sodium iodide were found to increase FI. This was likewise found with mercury arc illumination and hence they may prove useful for routine immunofluorescence tests. PPD was found to accumulate on the surface of the beads and to disturb immunofluorescence by autofluorescence. The value of any of the other reagents in immunofluorescence is questionable.