Native 1 alpha 2 alpha 3 alpha collagen (500 micrograms per rat) was both immunogenic and arthritogenic in Alderley Park rats (46% developed arthritis) but only immunogenic in Sprague-Dawley rats. Conversely, native type II collagen (500 micrograms per rat) was immunogenic and arthritogenic in both strains (64% arthritic in Alderley Park strain, 57% arthritic in Sprague-Dawley strain). The inflammatory polyarthritis induced by 1 alpha 2 alpha 3 alpha collagen was similar to that produced by native type II collagen in clinical appearance, time of onset, and histology. Antibodies raised to native bovine type II collagen cross-reacted with native 1 alpha 2 alpha 3 alpha collagen and vice versa. Thus the minor collagen component of cartilage, the 1 alpha 2 alpha 3 alpha collagen, as well as the major collagen component, type II collagen, are immunogenic and arthritogenic in the rat, with strain differences.