We report the sequences of more than 40 partial and complete swine C gamma cDNAs obtained by PCR cloning of first strand cDNA, and from a cDNA expression library, all from a single animal. These seem to represent five IgG subclasses, that can be grouped into two clusters; one contains IgG1 and IgG3 and the other, IgG2a, IgG2b and IgG4. IgG2a and IgG2b differ by only three amino acids, but single strand conformational polymorphism analyses of PCR-amplified IgG2-specific segments in animals of different breeds, argues for their putative subclass status. Major subclass differences are found in the hinge and C gamma 3, but differences in upper hinge length, associated with segmental flexibility in the IgGs of other species, are absent. All subclasses have identical middle hinge segments that can accommodate three interheavy chain disulfide bridges. The putative swine IgG subclasses have their greatest similarity with those of the human except for the near absence of hinge region variation. Swine subclasses such as ruminant IgG2a, have a lower hinge deletion which in human IgG1, contains one of the motifs believed necessary for interaction with Fc gamma Rs. IgG1 was the most frequently encountered subclass cDNA (25 of 43) and the single swine-mouse hybridoma tested had a sequence identical to IgG1. Partial sequence analyses of genomic clones identified one clone identical with the IgG1/IgG3 subclass cluster, two identical to the IgG2b/IgG4 subclass group, and two identical to each other but different from any of the expressed sequences reported here. Genomic blots suggest that up to eight C gamma genes are present in the genome.