The carboxy-terminal octadecapeptides from canine heavy chains of 1) IgGa, b (from normal serum) and 2) IgGd (from a paraprotein) were isolated. The amino acid sequences of both octadecapeptides were determined. Both peptides are clearly homologous to the same heavy chain region of IgG's from other mammals. Previously it has been shown that canine IgGd and equine IgG (T) share certain biologic properties; however, comparison of the sequences of their C-terminal octadecapeptides reveals no uniquely shared residues which would support a close phylogenetic relationship.