Serologic diagnosis of Chlamydia pneumoniae infection has been based on the microimmu-nofluorescence test (MIF). However, recent prospective studies in children have found that >50% infected with C. pneumoniae failed to develop any antibodies detectable by MIF. In this study, single sera from 46 culture-positive and 42 culture-negative children with respiratory infection and known MIF status were examined by immunoblotting. Forty-one (89.1%) of the single sera from culture-positive and 27 (64.3%) from culture-negative children reacted to C. pneumoniae antigens in immunoblot. C. pneumoniae proteins most frequently recognized by sera from culture-positive patients were at 101-102, 72-76, 50-52, 48-49, 43-44, 41-42, and 30-31 kDa. However, there did not appear to be a correlation of specific band patterns and culture status.