Of the four human IgG subclass heavy chain (IGHG) genes, three--IGHG1, IGHG2 and IGHG3--have allelic forms which encode antigenic determinants called G1m, G2m and G3m allotypes for markers of the IgG1, IgG2 and IgG3 molecules. The alleles at these three closely linked loci are transmitted as Gm haplotypes, together with the IGHG4 and the IGHGP (or pseudo gamma) genes. Restriction Fragment Length Polymorphisms (RFLPs) have been described for all these IGHG genes, allowing us to define at least 15 RFLP alleles (N. Ghanem et al., Eur. J. Immunol. 1988. 18:1059). In this article, we determine unambiguously the Gm alleles and haplotypes and the associated RFLPs in French, Lebanese and Tunisian families, chosen for the various Gm phenotypes of their members, in order to correlate both these polymorphisms and to know the evolution of the IGHG multigene family. We were able to assign all the IGHG alleles to the Gm alleles and to their linked IGHG4 and IGHGP genes allowing us to define restriction fragment haplotypes of the IGHG gene family.