SummaryDuring the course of megaloblastic anaemias the rise of the serum lactate dehydrogenase and the alteration of its isoenzymes are constant. The persistance of these alterations (particularly the zymogram) several weeks after the administration of vitamine B12, justifies the systematic evaluation of the SLDH and its isoenzymes in the diagnosis of any anaemia wether already treated or not. The interest of these measurements, in cases where the macrocytic character of the anaemia is masked by other existing deficiencies, is raised.