A prospective comparison was conducted in 43 patients to assess comparatively the ranking of cytology and CEA determined in the pleural effusions and by immunocytology on the cell surface (the latter two compared with each other) and then juxtaposed with the information obtained by histology via thoracoscopy. It was found that the yield achieved by cytology was comparable with that obtained by immunocytology. The maximum sensitivity was established with the adenocarcinoma.