Ponceau 2R staining on semi‐thin sections of tissues fixed in glutaraldehyde‐osmium tetroxide and embedded in epoxy resins

SUMMARY Proteins were stained (brilliant red) by using 0·5% Ponceau 2R (C.I. 16150) in sulphuric acid (pH 2) at 40°C on sections 1–2 μm thick of tissues fixed in glutaraldehyde‐osmium tetroxide and embedded in epoxy resins. The staining is free from precipitates and does not require the removal of the embedding media. Section bleaching with 1–2% periodic acid at 40°C for 30–90 min was also carried out in order to obtain a more brilliant colour of the proteins which may be photographed using black and white films.