We have determined lactic acid in a number of serous fluids and in samples of blood taken at the time of paracentesis. Determinations were made immediately by the method of Clausen.The exudates all showed a considerably greater lactic acid content than that of the blood, the highest values and greatest excess being found in frankly purulent fluids and in sero-fibrinous exudates (streptococcus) which later became purulent. The transudates yielded relatively low figures, usually slightly below the blood values.The accumulation of lactic acid in inflammatory exudates may be due to bacterial action or to the action of leucocytes on glucose and other metabolites, as noted by Levene and Meyer.1 In malignant exudates its presence, in concentrations greater than that of the blood, would be explained by the observation of Warburg,2 whose study of the respiration of malignant tissues showed a production of lactic acid by cancer cells many times greater than that of normal tissues.