1. Isolated canine haptoglobin behaved identically to the alpha 2 beta 2 structure typical of human haptoglobin type 1-1 on alkaline polyacrylamide gel electrophoresis and on gel filtration. 2. In the presence of urea or sodium dodecyl sulphate canine haptoglobin dissociated into alpha beta subunits that separated into alpha and beta chains after reduction with 2-mercaptoethanol. 3. Compositional analysis identified one less half-cystine in canine alpha chain when compared to human alpha 1 chain. 4. These results provide evidence that there is no inter alpha chain disulphide in canine haptoglobin comparable to the alpha 1 20-alpha 1 20 disulphide in human haptoglobin that links the two alpha beta subunits.