Fifty adult Japanese patients with biopsy-proven idiopathic membranous nephropathy (IMN) were typed for HLA-A, B, and DR antigens, and additional B cell alloantigens designated the MT system. The phenotype frequencies of HLA-DR2 (80.0 vs. 36.0%) and MT1 (96.0 vs. 49.5%) appeared to be significantly increased in the patient group when compared to those of a control. There were lower frequencies of DR4, DRw9, MT3, and MT4 in these particular patients. The decreases of MT3 and MT4 were statistically significant but those of DR4 and DRw9 were not. These results suggest that HLA-DR2 and MT1 antigens are related to the occurrence of IMN in the Japanese population.