A procedure to detect Chlamydia in postmortal formalin-fixed tissue is described. Monoclonal antibodies against a genus specific chlamydia epitope were used in immunofluorescence to detect chlamydia inclusions in formalin-fixed tissue sections. Lung sections from chlamydia-infected mice were examined and the effect of autolysis and tetracycline treatment was evaluated. Furthermore, lung tissue from two patients who died of ornithosis was examined. Inclusions detected in lung sections showed a bright apple-green fluorescence, and had a characteristic and easily recognizable morphology. Background and non-specific fluorescence were reduced by treating the tissue sections with trypsin, rabbit serum and Evans blue counterstain. Besides giving an exact diagnosis at autopsy, the method provides the possibility of determining the occurrence of chlamydia infections in various tissues, based on retrospective investigations in formalin-fixed tissues.