Two cases of Chlamydia psittaci pneumonia are presented. In each, a rapid diagnosis was made through the use of direct immunofluorescent antibody staining of respiratory secretions with monoclonal antibodies to chlamydial antigens. In one case the diagnosis was confirmed by the isolation of the causative organism from sputum and a pharyngeal swab. Chlamydial lipopolysaccharide was detectable in sputum from this patient in an enzyme immunoassay. Serological responses to C. psittaci, Chlamydia trachomatis, and Chlamydia pneumoniae were evaluated, and serological cross-reactivity was observed between each species. Rapid antigen detection systems for Chlamydia species that use commercially available reagents can be helpful in the evaluation of selected patient populations.