Frequencies of the HLA antigens ABC, DR and MT, as well as of the properdin factor alleles were determined in 24 unrelated patients presenting with immune complex mediated idiopathic rapidly progressive glomerulonephritis (RPGN) type II. As in Goodpasture syndrome (RPGN type I with pulmonary hemorrhage), a significant association with the B-cell alloantigen HLA-DR 2 was demonstrated (relative risk for HLA-DR 2 positive individuals was 3.54; P less than 0.01). In addition a marked increase of the HLA specificity MT 3 was shown, which is supposed to belong to an antigen system of a second HLA-D region locus. The highest relative risk of 14.67 (P less than 0.00001), however, was calculated for all patients carrying the BfF phenotype. Increased numbers of patients positive for HLA-DR 2 and -MT 3, as well as BfF suggested immune response genes or disease-related mutations on different haplotypes responsible for a MHC (major histocompatibility complex) associated predisposition of RPGN type II.