Sequence analysis of 84 V region cDNAs expressed with IgM, IgG and IgA from both adult and newborn swine suggested that their JH segments had been derived from the same germline JH. Only a single hybridizing JH segment could be identified in genomic DNA, in a JH-C mu cosmid and in a Sacl fragment of the cosmid extending 5 kb 5' of the EnhH. The single germline JH segment mapped 6 kb 5' to C mu. This JH had a sequence identical to 40 of 42 JH segments expressed in a newborn piglet and 25 of 42 expressed by adult swine. None of the 19 JH segments which varied from the germline sequence were identical to each other and half of the nucleotide changes were silent. No cosmid DNA hybridizing with heterologous probes for C delta could be found within 20 kb 3' of C mu and C delta could not be cloned from genomic or cDNA libraries. A conserved IgD fragment could be amplified from human, mouse and rat genomic DNA but not from rabbit, swine or cattle. We hypothesize that heavy chain organization and constituency in homeothermic vertebrates is correlated with the site of secondary antibody repertoire development and the mechanism(s) used.