The genetic polymorphism of eight red cell antigenic systems was examined in two population samples from Cantabria (north Spain). No significant differences were found between the autochthonous and the resident populations. The results obtained are compared with those for other populations in the north of Spain. The low frequency of the B allele of the AB0 system and the high frequency of the 'cde' haplotype of the Rh system in the Cantabrain and Asturian population suggests that the anthropological characteristics of these two systems, which hitherto have been attributed to the Basques, also apply to the western neighbouring regions.