In Western blotting, Ab-3 reacts with the TCA fragment of lathyritic type II collagen after digestion with mammalian collagenase. It also reacts with pepsin- digested type II collagen. Immunoblotting of CNBr peptides of collagen II shows that MAb 6B3 reacts with CB 11 fragment which is the site of immunogenic and arthritogenic epitopes along the intact type II molecule. In Western blotting (but not ELISA), Ab-3 recognizes α1(II) as well as α3(XI) chain both of which have identical primary structure.