We determined the levels of lysozyme in pleural fluid and serum in 141 patients with the following different causes for their pleural effusions: tuberculosis; neoplasias; transudates; parapneumonic, not complicated; empyemas; and miscellaneous. The lysozyme level of the pleural fluid and the ratio of that level over the serum level of lysozyme (PL/SL ratio) was meaningfully increased in patients with empyema (p less than 0.01). The groups with tuberculous and neoplastic effusions showed significant differences in the PL/SL ratio (p less than 0.01). The existence of a raised PL/SL ratio suggested important local synthesis of lysozyme, and it came up in empyemas and tuberculosis, unlike the other groups. Excluding the patients with empyemas, a PL/SL ratio of 1.2 showed a sensitivity of 100 percent, specificity of 94.9 percent, positive predictive value of 94.7 percent, negative predictive value of 100 percent, and accuracy of 97.3 percent for the diagnosis of tuberculous pleural effusion. All of this suggests that the determination of the lysozyme level can be an easy method of great usefulness in the initial diagnosis of pleural effusions.