Genetic polymorphism at Mhc class II DRB loci was investigated in samples of musk-ox from Canada and Greenland; moose from Sweden, Norway, Canada, and Alaska, roe deer from Norway and Sweden; reindeer from Svalbard and Norway; fallow deer from Norway and Sweden; and red deer from Norway. The results were compared with published data on cattle, bison, goal, sheep, and red deer. Cattle-specific primers amplified a single DRB locus in all species except fallow deer and red deer, in which two loci were found. Single strand conformation polymorphism analysis and DNA sequence analysis were employed to detect genetic polymorphism. Complete monomorphism was found in musk-ox and fallow deer. Limited polymorphism was found in the moose, roe deer, and reindeer from Svalbard, whereas intermediate to extensive DRB diversity was present in reindeer from Norway and in bison, sheep, goat, cattle, and red deer. The restricted Mhc diversity in moose, roe deer, and fallow deer is notable in relation to the dramatic population expansion of moose and roe deer in Sweden during this century and since fallow deer is used for meat and game production with good results and without any marked disease problems. The results question the view that species or populations with restricted Mhc diversity have poor resistance to infectious diseases. A phylogenelic tree analysis revealed a clustering of DRB sequences within species rather than within allelic lineages across species. The results suggest trans-species persistence of polymorphic sequence motifs rather than of allelic lineages.