A commercial test (rELISA) based on a recombinant chlamydial lipopolysaccharide (LPS) antigen has been evaluated for the diagnosis of acute infections caused by Chlamydia pneumoniae (TWAR) and Chlamydia psittaci . This test and a microimmunofluorescence test (MIF) were compared in 160 patients with community-acquired pneumonia. Seventeen of nineteen cases with significant titre changes detected by rELISA were confirmed by MIF. The two remaining cases not confirmed by MIF were considered false-positive reactions. One case positive by MIF only was judged not to be a true-positive reaction. All three cases occurred in patients with Mycoplasma pneumoniae infection and may be the result of a mitogenic effect. High antibody titres have been used to indicate acute C. pneumoniae infection. We found high MIF or rELISA titres to be equally common in patients and controls; no association between the two tests was detected. An unexpected cross-reactivity between the rELISA antigen and parvovirus was observed, which might have diagnostic implications. Both MIF and rELISA detected acute C. pneumoniae and C. psittaci infection, and there was good agreement between the tests. Single serum diagnosis was generally not feasible with either MIF or rELISA.