Multigene families are known to be evolving under continued occurrence of unequal crossing-over and gene conversion, and MHC loci are no exceptions. Population genetic models of diversifying selection, with intra- and inter-locus gene conversion were studied by computer simulations. Weak selection at individual amino acid site is shown to be quite effective in enhancing polymorphisms, if gene conversion occurs. This is caused by the appearance of coadapted segments of mutations in which recombination tends to be suppressed. In addition to the global type of diversifying selection, endemic type of selection was introduced in some sets of simulations to find out a mechanism of the rapid allelic turnover of South American Indians. Strong selection is assumed such that the most common two alleles become lethal in one generation. If this endemic selection occurs periodically, allelic turnover by intralocus gene conversion is shown to become rapid. Hence, interaction of selection and gene conversion has a profound effect on MHC polymorphisms.