An oil emulsion of purified type II collagen from bovine articular cartilage when injected intradermally into rats induced an inflammatory polyarthritis in 4 of 12 animals. When similarly injected, collagen purified from bonve vitreous induced arthritis in 6 of 12 animals. Studies of humoral and cell-mediated immunity to both collagen preparations demonstrated complete cross-reactivity. It is concluded that vitreous collagen shares the arthritogenic property of cartilage-derived type II collagen and that collagen from the two sources is indistinguishable in arthritogenic and immunologic properties.