An unusually heavy load of Epstein-Barr virus (EBV) infection and autoimmunity to collagen are believed to be contributing factors to the pathogenesis of rheumatoid arthritis (RA). The present report presents data showing that p107, the major epitope of the EBV-encoded EBNA-1 antigen, cross-reacts with denatured collagen (DC) and keratin (K), suggesting a new likely link among RA, EBV-1, and these autoantigens. A radioimmunoassay using antigen-coated microtiter plates was used to demonstrate antibodies in sera of patients with RA and sera of healthy donors against p107, DC, and K. Specificity of the antibodies was ascertained by inhibition tests with the homologous antigens. Cross-reactivity among anti-p107, anti-DC, and anti-K antibodies was assayed by the ability of a given antigen to block the binding of nonpurified or affinity-purified antibodies to plates coated with another antigen. Most of the sera contained antibodies to all three antigens, but only anti-DC antibodies were present in higher titers in RA sera. Preincubation of sera with p107 appreciably reduced their binding to plates coated with DC or K. On the other hand, preincubation with DC (in solution or bound to Sepharose) did not result in consistent reduction of anti-p107 titers. Tests with affinity-purified antibodies revealed the existence of two antibodies populations, one of which reacted preferentially with p107, the other with DC. The cross-reactivity of the anti-p107 antibodies with DC and K suggests that such antibodies, produced by RA patients following persistent stimulation with EBV, might react in vivo with collagen (and keratin) exposed in previously damaged areas and thus reinforce the disease process.