In 96 patients with pleural exudates of known origin the authors assessed different cytological, biochemical and immunological parameters in the fluid and serum. In 44 patients the pleural effusions were malignant, in 26 tuberculosis, in 17 bacterial and 9 of viral nature. Statistical analysis was carried on mean values of studied parameters. The results indicate that only bacterial effusions differ significantly from the other pleural effusions. In effusions of malignant, tuberculous and viral nature the studied parameters did not differ significantly and cannot be used as differentiating criteria.