Antibodies to native types I, II, IX, and XI collagen were measured, using a 125I-solid-phase radioimmunoassay, in serum from 104 patients with rheumatic diseases (rheumatoid arthritis, osteoporosis, Paget's disease, or osteoarthritis). In all disease groups, antibodies to type II collagen occurred with greater frequency than antibodies to type I collagen (11-35% versus 5-23%). Antibodies to type XI collagen were the most frequent: They were present in approximately 50% of the patients in the rheumatoid arthritis, Paget's disease, and osteoporosis groups. Antibodies to type IX collagen were found at a high frequency in the rheumatoid arthritis group only (44%). Analysis of the clinical data suggested that the presence of antibodies to collagen was associated with disease that was less severe or of shorter duration.