We have identified DNA sequence polymorphisms in the Cγ3 genes of BALB/c and C57BL/6 mice. One of these results in a Ser→Gly amino acid difference in CH1 at position 129 according to the Wu and Kabat numbering system. There are three additional silent substitutions in the coding region and two polymorphic nucleotides in the 3' untranslated region. According to standard nomenclature in which alleles are numbered according to the order of their identification, these Cγ3 alleles are designated lgh-8a and Igh-8b respectively. We also describe two polymerase chain reaction-based assays that identify the allelic differences. © 1995 Oxford University Press.