An improved enzyme-linked immunosorbent assay (ELISA) for the detection of anticollagen antibodies in human serum has been developed. With the use of this method, antibodies against native human Type II collagen were detected in 22.7% of sera from 480 patients with rheumatoid arthritis (RA). The antibodies were found to be collagen type specific, showing no reaction with human Type I and Type III collagens. The antibodies appeared in high incidence during the early phase of the disease, and RA patients with involvement of a single joint, mono-articular arthritis, were often positive for anti-Type II collagen antibodies. In most of these patients, anti-Type II collagen antibodies preceded the appearance of rheumatoid factors. The antibodies were all negative in sera from patients with gout, osteoarthritis (OA) and non-arthritic diseases. Thus, anti-Type II collagen antibody assay may have diagnostic significance for RA patients, especially those in whom laboratory and clinical findings provide only minimal help in diagnosis.