In order to understand the clinical importance of Haemophilus influenzae isolated from sputum samples, an indirect immunofluorescence (IF) assay was developed, using the patient's own isolate as the antigen. The method was tested on samples from six patients with lower respiratory tract infection (LRTI) and H. influenzae isolated from blood (n=2), sputum (n=3) or both (n=1), and on two healthy adults with H. influenzae isolated from the nasopharynx. Between acute and convalescent sera, a four-fold IgG antibody increase was achieved in five of six LRTI patients, including the three blood culture-positive patients. One LRTI patient and the two asymptomatic carriers showed stable antibody levels against their own isolate. Although small, the study indicates that indirect IF can be a promising tool for determining whether a H. influenzae strain represents the probable cause of infection or just a strain colonising the airways. More extensive studies should be performed in order to establish the usefulness of the assay.