Branhamella catarrhalis is now recognized as an important cause of lower respiratory tract infections, especially in the elderly. In most instances, pathogenicity is presumed by recovery of the organism in a sputum culture, a method that is less than conclusive. In order to better diagnose B. catarrhalis infections, an enzyme-linked immunoassay has been developed using P-protein as antigen to measure antibodies to B. catarrhalis. In 17 elderly patients with B. catarrhalis pneumonia and 12 with tracheobronchitis, acute-phase serum antibody titers to P-protein were found to be significantly increased when compared with those of normal subjects (both p less than 0.02). There were no differences in antibody titers between patients with pneumonia and tracheobronchitis. Antibody titers of convalescent-phase sera increased over those of acute-phase sera in 46 percent of pneumonia patients and 50 percent of tracheobronchitis patients. The results demonstrated that lower respiratory tract infections with B. catarrhalis promote a significant elevation in antibody response to P-protein of B. catarrhalis.