A modified glucose oxidase immunoenzyme technique was shown to be highly sensitive and specific for detection of serogroup 1 Legionella pneumophila in 4% formaldehyde solution-fixed, paraffin-embedded tissue sections. There was complete concordance between infection with L pneumophila and detection of the organisms in tissue sections by glucose oxidase immunoenzyme staining. The L pneumophila organisms stained blue-black and were found within phagocytic cells as well as in the extracellular space. A cloud of blue-black pigment, probably representing diffusable antigen, was present in the extracellular spaces in the area of L pneumophila localization. No false-positive or false-negative reactions were found. This technique requires no specialized equipment, may be applicable to retrospective diagnostic problems, and can be adapted to routine diagnostic practice.