IgG subclass-specific antibodies to Chlamydia pneumoniae and chlamydial lipopolysaccharide (LPS) were analysed in paired sera obtained from 15 patients with primary C. pneumoniae pneumonia and from 16 pneumonia patients with reinfection, as well as in single sera of 40 subjects with possible chronic C. pneumoniae infection and 40 healthy controls. The microimmunofluorescence (MIF) method was used to measure total IgG, IgM and IgG subclass-specific antibodies to C. pneumoniae protein antigens and enzyme immunoassay (EIA) to measure antibodies against the LPS antigen. By MIF, IgG1 antibodies to C. pneumoniae were demonstrated in all individuals of the 3 patient groups and also in all healthy controls. IgG2 subclass antibodies were not found by MIF. IgG3 antibodies were detected in 40% of patients with primary infection, in 31% of patients with reinfection, in 25% of those with chronic infection and in 8% of the controls. IgG4 antibodies were associated with acute C. pneumoniae infection and were found in 13% of primary infections and 31% of reinfections. The subclass pattern of LPS antibodies resembled that of protein antibodies measured by MIF: IgG1 was the most common subclass among the antibodies to LPS.