Paired sera from 97 patients with acute maxillary sinusitis were examined regarding antibodies to Branhamella catarrhalis. Precipitating antibodies were demonstrated in almost all sera both from patients and from healthy blood donors. Complement-fixing (CF) antibodies to B. catarrhalis were present in sera from 25 of the 97 patients and in one of 20 healthy blood donors. The titres were low and the titre changes when present were of a small magnitude. CF antibodies were most commonly demonstrated in the younger age groups. The patients with demonstrable CF antibodies to B. catarrhalis did not differ from patients without antibodies regarding radiological appearance or healing during therapy. Strains of B. catarrhalis were all rapidly killed by normal human serum but not in heated sera. The strains could not multiplicate significantly at an oxygen tension corresponding to about half the atmospheric value. The possible significance of the serological and bacteriological findings is discussed.