Ninety-five rheumatoid arthritis patients treated with aurothiomalate and/or D-penicillamine have been studied for possible associations between HLA-A, -B, -DR antigens and various toxic reactions to the above drugs. HLA-DR3 and -DRw6 had a higher frequency in patients with toxic reactions (all types) than in patients without toxic reactions (28.5 per cent vs 13.0 per cent and 26.5 per cent vs 4.3 per cent, chi 2 = 2.6 and 7.2, respectively). HLA-B8 was found at a higher frequency in patients with proteinuria and other types of renal involvement (20.0 per cent vs 7.4 per cent in controls), whereas skin manifestations were mainly associated with the presence of HLA-DRw6. The lowest frequency of side-effects was seen in patients with HLA-DR1 and DR2 (10.2 per cent vs 28.3 per cent and 28.5 per cent vs 54.3 per cent, chi 2 = 3.9 and 5.5, respectively). In addition, seropositive patients possessing HLA-DR1, showed toxic reactions less frequently.