The prevalence of a new polymorphism for the restriction enzyme Ava II in the psi beta-gene of the beta-globin gene cluster was determined in Mediterranean families with at least one beta-thalassaemia homozygote. The polymorphic site was absent in 54/115 beta-thalassaemic chromosomes but only in 4/120 normal chromosomes. The difference in frequencies of this polymorphism between normal and thalassaemic chromosomes greatly increases the feasibility of prenatal diagnosis of beta-thalassaemia by DNA analysis in this population.