Raised serum IgG and IgM antibody levels to native type II collagen were found in patients with rheumatoid arthritis and in patients with juvenile chronic arthritis. There was a good correlation between the serum IgG and the IgM antibody levels in rheumatoid arthritis and a weaker correlation in juvenile chronic arthritis. Raised serum IgM antibody levels to native type II collagen were found in only 1 patient each with ankylosing spondylitis and psoriatic arthritis, and in these groups here was no correlation between the serum IgG and the IgM antibody levels. The highest serum IgG and IgM antibody levels to native type II collagen were found in rheumatoid arthritis. These results, together with the results on serum antiglobulin levels, indicate that patients with rheumatoid arthritis produce antibodies of both IgG and IgM immunoglobulin class which may have pathogenetic significance in the more severe arthritis found in this condition.