Analysis of serum and BAL fluid immunoglobulin levels in individuals with PBD and in asymptomatic but similarly exposed pigeon breeders was carried out by immunofluorometric assays. The results indicate that the group with PBD have significantly higher levels of IgG and IgA in their BAL fluids but that IgM levels were not significantly different in the two groups. These differences were not reflected in the serum immunoglobulin levels of the two groups. The elevated BAL fluid IgG levels in individuals with PBD is associated with an increase in IgG4 subclass levels as determined PHA inhibition. These studies suggest a role for this subclass in the pathogenesis of the disease.