Hb G-San Jose [alpha 2 beta 2 7(A4)Glu----Gly] was detected in four families and three unrelated individuals from Eastern Sicily. Polymorphic restriction sites within the beta-globin gene cluster bearing the mutation were characterized. A complete association of beta-G-San Jose alleles with Mediterranean haplotype IV was found in the families examined and the same haplotype was also present in the unrelated individuals. These findings support the hypothesis of an unicentric origin of the beta-G-San Jose mutation which may have arisen in Eastern Sicily.