Previous testing of patients with rheumatoid arthritis showed that one HLA-D type, Dw4, occurred more frequently than in normal controls. B-cell alloantigens closely related to HLA-D can now be identified by a simple serologic procedure. Using this test, I studied 80 white patients with erosive, rheumatoid-factor-positive rheumatoid arthritis. The B-cell alloantigen HLA-DRw4 occurred in 70 per cent of 54 patients, as compared to 28 per cent of the 68 normal controls (P less than 10(-5)). Both groups were also tested for the HLA-A, B and C antigens and for HLA-D. HLA-Dw4 occurred in 54 per cent of the patients and 16 per cent of the controls (P less than 10(-5)). Small differences observed in several of the HLA-A and B antigens were not statistically significant. The results indicate that rheumatoid arthritis in whites is associated with genes of the HLA-D region and that immunogenetic factors linked to HLA have a role in its pathogenesis.