Lactate concentrations in the pleural fluid of 50 patients were determined by the Monotest Lactate Kit. Lactate values were found higher in bacterial pleural infections than in cases of hydrothorax. Elevated levels were also found in most cases of histologically confirmed cases of pleural neoplasms and in some cases of non-bacterial pleuritis exsudativa. The highest levels were found in cases of empyema, but there was considerable overlapping between the groups. Pleural lactate thus appears to have little diagnostic value in the differential diagnosis between empyema and non-bacterial pleural effusions.