Associations between HLA types and serum antibodies to native and denatured type II collagen were sought in 105 patients with rheumatoid arthritis (RA). Antibodies were measured using a solid phase radioimmunoassay. There were no significant associations between any HLA antigen (A, B, or DR) and a high antibody titre to native collagen. There were significant associations, however, between HLA antigens and high antibody titres to denatured collagen. Although DR4 did not show an association, the phenotype A2+DR4+ did; this was not related to A2 as A2+DR- was not associated with a high antibody titre. No single B locus antigen showed an association, but several B locus antigens, B12, B15, and B40, were included in phenotypes with A2 and DR4 which were associated with a high antibody titre to denatured collagen. These HLA associations with anticollagen type II are best explained by a gene other than DR4 (but in linkage with it) which may regulate the antibody response to denatured collagen. If so, this would represent an HLA gene in addition to DR4 that is active in RA.