Sputum samples obtained routinely for culture from patients at a thoracic department were also examined for pneumococcal antigen by means of counterimmunoelectrophoresis (CIE), using a polyvalent antipneumococcal type serum (omniserum). Pneumococci were found in 1.3% of the 880 cultures, whereas pneumococcal antigen was detected with CIE in 6.5%. The validity of these findings was tested by correlating them with the presence of clinical symptoms in those with positive tests and also by antigen detection in ELISA using monoclonal antibodies specific for the C-polysaccharide common to all types of pneumococci. Clinical findings corresponding to confirmed or probable current chest infection were found in 36 of the 48 patients with positive CIE. ELISA was positive in 33 of the 38 patients with positive CIE who were tested. Although the study deals with an unselected material of chest patients, it indicates that CIE is a sensitive method and that it is independent of current antibiotic treatment. Pneumococcal infection is probably of importance in exacerbations of chronic obstructive lung disease, but the clinical usefulness of detecting pneumococcal and other antigens in this patient group needs to be studied further.