To investigate the possible relation between certain HLA antigens and toxicity during treatment with sodium aurothiomalate of D-penicillamine, we studied 91 patients with rheumatoid arthritis. Seventy-one had toxic reactions to either drug or both drugs; the remaining 20 took one of the drugs for at least six months, without toxicity. Nineteen of 24 patients in whom proteinuria developed were positive for HLA-B8 and HLA-DRW3 antigens; 14 of 15 episodes of aurothiomalate-induced proteinura and nine of 13 episodes of penicillamine-induced proteinura occurred in patients with these antigens. All 13 episodes of proteinuria in which urinary protein exceeded 2 g in 24 hours occurred in patients with DRw3. The relative risk of proteinuria during treatment with aurothiomalate is increased 32 times in patients who are HLA-DRw3 positive. No significant associations were found between any HLA antigen and development of skin rashes or hematologic complications. Toxicity during aurothiomalate or penicillamine treatment for rheumatoid arthritis may be under genetic control.