1. Pleural fluid contained protein-bound hyaluronic acid, protein-bound chondroitin sulfate, hyaluronic acid, chondroitin sulfate, undersulfated chondroitin sulfate and dermatan sulfate. The composition of acid glycosaminoglycans in pleural fluid seems to reflect the rate of biosynthesis and degradation of these polysaccharides at some sites which are closely related to the pleural cavity. 2. A possibility was suggested that hyaluronic acid was synthesized in pleural tissue and was excreted shortly thereafter into the surroundings, as evidenced by experiments with rabbit pleural tissue. 3. In human, hyaluronic acid, chondroitin sulfate, dermatan sulfate and heparan sulfate were found in thickened pleurae caused by lung cancer, in those caused by asbestosis and also in tumor tissues of pleural mesothelioma. The molecular size of hyaluronic acid from pleural mesothelioma was found to be larger than that from human unbilical cord. 4. Quantification and histochemical study of acid glycosaminoglycans demonstrated that the quantity of hyaluronic acid in tissue specimens of mesothelioma by far exceeded that in non-mesothelioma cases (statistically significant). 5. Thus a possibility was suggested that histochemical investigation together with microquantitation of hyaluronic acid in pleural tissue may prove to be an efficient means of differential diagnosis of pleural mesothelioma. 6. Definite conclusion on the relationship between the fluctuation with time in quantity of acid glycosaminoglycans of the effusions and etiology of pleurisy awaits further investigations.