Among several hundred apparently healthy Yugoslavian adults with slightly elevated levels of fetal haemoglobin, we have identified two distinct abnormalities. (a) A G gamma A gamma(delta beta)0-thalassaemia heterozygosity with an approximately 15 kb deletion which involves part of the delta globin gene and the beta globin gene. This deletion is probably the same as that seen among Italians (Ottolenghi et al, 1982; Carè et al, 1984). (b) A nondeletion form of hereditary persistence of Hb F which is caused by a gamma globin gene triplication of the (+)G gamma.(+)G gamma.A gamma type. It is characterized by the presence of some 5% Hb F in the heterozygote containing nearly 100% G gamma chains. The C----T mutation at position--158 5' to the G gamma chain [(+)G gamma], identified through analyses of Xmn I digests, was present at both G gamma globin genes. This mutation is known to be associated with increased G gamma chain production (Gilman & Huisman, 1985), and thus is responsible for the increased G gamma chain production in these heterozygotes. The condition is different from the (+)G gamma.(+)G gamma nondeletion type of HPFH which has been observed in heterozygotes of two Black families, and is associated with the presence of 3-4% Hb F (with mainly G gamma chains) in heterozygotes.