Osmium tetroxide and OTAN histochemical reactions have been reappraised following recent publications in this journal by Ellbder and Lojda. Provided that the standard OTAN reaction is used on frozen sections of conventional thickness, unsaturated hydrophilic polar lipids are stained in an orange or red shade while unsaturated hydrophobic non-polar lipids are stained brown-black or black. We have confirmed the anomaly — pointed out by Elleder and Lojda — that the “polar lipid” reaction of the atherosclerotic plaque is essentially extinguised by acetone. Solubility and blockading methods provide circumstantial evidence that Elleder and Lojda's assumed OsO4-protein reactions are due to the lipid moiety of a lipoprotein complex.