The first step in the diagnosis of a pleural effusion is to determine the exudative or transudative nature of the fluid. The purpose of this work was to compare different biological criteria commonly used in clinical practice. Among 118 pleural effusion fluids studied, 24.6% were exudates and 75.6% were transudates. The different parameters studied were Light criteria, Costa criteria, protein content, cholesterol content, and lactate dehydrogenase activity in the pleural fluid. The results showed that the Light criteria enabled classing all the fluids as exudates or transudates. We were also able to demonstrate that simple assay of lactate dehydrogenase activity in the pleural fluid gave comparable results, avoiding the need for further blood samples.