HLA-A, B, DR, and MT antigens were examined in 50 patients with idiopathic membranous nephropathy (IMN) by the standard microlymphocytotoxicity technique. The frequency of HLA-DR2 was significantly increased in the patients in comparison to the controls. Significant decreases in the frequencies were seen for HLA-DR4, DRw8 and DRw9 in the patients. The linkage disequilibrium between HLA-Bw52 and DR2 was noted in the controls, but it was significantly weaker in the patients. The frequency of MTl was significantly higher, and that of MT3 was significantly lower in the patients as compared with the controls. The findings suggest that an immunological disturbance under the influence of genetic factors may confer a susceptibility to IMN.