ALTHOUGH the isolation of ergosterol peroxide from the extracts of several different fungi has been reported1–4, the question of its authenticity as a metabolite in these cases does not seem to have been raised. In the present case, the peroxide was found to be present in extracts of sporophores of Piptoporus betulinus or of Daedalea quercina. These extracts, however, had been exposed to daylight for several days and the peroxide could not be detected initially in fresh extracts. Ergosterol (RF 0.40) and ergosterol peroxide (RF 0.23) were both easily detected by T.L.C. on silica using 1 per cent methanol in chloroform as eluent, characteristic intense blue–black and dark green colours, respectively, being developed after spraying with coric ammonium nitrate and heating briefly at 100° C. A sample of the peroxide isolated after chromatography on silicic acid was identical in all respects with a synthetic sample5.