187 pleural fluids of various etiology are examined with the intention to find hyaluronic acid. The colorimetric method of Morgan-Elson was used for this purpose. The mucopolysaccharid was identified in 34 cases: 18 in 30 cases of diffuse mesotheliomas 1 in 74 cases of secondary tumours of the pleura 8 in 20 cases of tuberculous pleurisy 5 in 37 cases of nonspecific pleurisy 2 in 6 cases of purulent pleursy. A level from more than 212 mg/litre was detected in 10 cases of mesotheliomas only. Such a concentration is consequently the proof for diffuse mesothelioma. The diagnostical improvement in other cases of mesotheliomas resulted from a combination of the following parameters: coagulation of protein by the method of Weltmann, cytological investigation of cellsedimentation and measurement of hyaluronic acid. Our experience with the diagnosis of diffuse mesotheliomas is discussed also in account to the results which are published since 1965 from a scientific team working in Lille, France, reporting the value of detection and measurement on hyaluronic acid in pleural fluids.