Some paint layers from Roman mural paintings coloured with red and purple haematite, from the acropolis of Lero (Alpes-Maritimes, France) were examined with electron microprobe. The coloration of the paint layer depends on the granulometry of the haematite for the tint and on the haematite concentration in the very top surface for the saturation. In each layer, the haematite used was pure and not an ochre. Bibliography, plates and diagrams.