In order to study the mechanism of autoantibody formation to type II collagen in rheumatoid arthritis (RA), IgG and IgA antibodies in sera from 259 RA patients and 285 non-RA controls were evaluated for their specificity as to collagen type (I and II) and species (chick, bovine, and porcine) using an improved enzyme-linked immunosorbent assay. IgG and IgA anti-type II collagen antibodies were commonly found in both RA (IgG, 41%; and IgA, 45%) and non-RA (IgG, 36%; and IgA, 31%) sera. Both IgG and IgA collagen antibodies were highly reactive with one or more heterologous type II or type I collagen; however, approximately 35% of IgG and 50% of IgA antibody-positive sera from both RA patients and non-RA controls cross-reacted with human type II collagen (HII) to some degree. However, no antibodies specific to HII were observed in either RA or control sera. In individual patient sera, IgG and IgA antibodies had identical collagen-type and species specificities. Importantly, IgG anti-HII antibodies purified from RA sera by affinity chromatography reacted equally with human, chick and bovine type II collagens, suggesting reactivity with conserved epitopes shared by all three species. In contrast, purified IgG anti-HII antibodies from non-RA control sera commonly lacked reactivity with one or the other of the heterologous type II collagens, suggesting reactivity limited to epitopes shared by HII and only one of the heterologous type II collagens. These data suggest that dietary collagens could elicit circulating IgG and IgA anti-collagen antibodies that cross-react with autologous type II collagen. Also the epitope specificity of IgG autoantibodies may be relevant to the pathogenesis of RA.