52 symptomatic (SPB) and 64 asymptomatic (APB) pigeon breeders were investigated for the HLA-A, -B, -C, -DR; C2, C3, C4 and Bf systems, and C3, C4 and factor B serum concentrations. HLA-DR3 and Bf S frequencies were significantly higher in the SPB than the APB group. Mean factor B concentrations were lower in the SPB than in the APB group. A positive two-way association between HLA-DR3 and the disease was found. It is concluded that a gene or genes responsible for type III and IV allergic reactions leading to the disease is associated with HLA-DR3. The increase of Bf S allotype and low mean factor B concentrations, however, can be explained by the strong linkage disequilibrium between BfS and HLA-DR3.