In four different popula tions of Spain, the Basques, the Galicians, the Andalusians and in the population of Central Meseta, the genefrequencies of the following geneticmarkers in red cells have been determined and compared: acid phosphatase, adenylate kinase, adenosine deaminase, phosphoglucomutase (locus 1) and 6-phosphogluconate dehydrogenase.