Lactic acid dehydrogenase (LDH) activity and protein content were studied in 80 patients with pleural effusion. Cases were categorized into malignant, infective, and transudative groups. In all of the 29 patients with inflammatory effusion and in 22 out of 24 patients with malignant effusion, pleural fluid LDH levels were above 550 units. In none of the 27 patients with transudative effusions were the LDH levels above 550 units. All malignant and inflammatory effusions had protein content above 3 gm/100 ml. In 19 out of 27 patients with "transudative" effusions, the plural fluid protein content was above 3 gm/100 ml. The results indicate that the pleural fluid LDH activity of 550 units serves better than protein in differentiating exudative effusions from transudative effusions.