Contamination of thin sections: Some observations on the cause and elimination of “embedding pepper”

A method is described for eliminating a form of contamination called embedding pepper that appears in, or on, thin sections following poststaining in lead citrate. The pepper is present most often in the matrixes of mitochondria, peroxisomes, and red blood cells. Formation of pepper can be prevented by treating the section with 0.5 N HCl or 1.0% ethylenediaminete-traacetic acid (EDTA) for 1–5 min before staining in uranyl acetate or lead citrate.