The minimal gene diversity at a locus of the antibody constant region, as estimated in natural populations of rabbit, revealed levels of heterozygosity similar to those reported for the major histocompatibility complex in human and murine populations. Sera of 416 wild rabbits were collected on the Iberian peninsula and on three islands of the Azorean archipelago and analyzed for the occurrence of the serological markers of the b locus of the immunoglobulin light chain. All four serotypes present in domestic rabbits were found in Portugal. They represented less than 50% of the gene pool. In Andalusia this was less than 15% and on the Azorean islands less than 10%. The pronounced and systematic hierarchy in allele frequencies, previously found in populations from the more recent distribution area of the species, was not observed. On the peninsula, the frequencies of the ``domestic'' alleles were similar, averaging 10%. The Portuguese sample revealed a total heterozygosity of at least 87%. This high value was supported by at least 11 serologically different alleles, none of them occurring at frequencies above 20%. These data are in agreement with an Iberian origin of the European rabbit and strongly suggest the coalescence of b locus allelic lines drawn from Iberian and western populations. The role of balancing selection in the evolution of the b locus polymorphism was further emphasized.