Following transurethral prostatic resection in 69 patients, the haemoglobin content of the irrigation fluid was determined first after manual stirring, then after mechanical stirring, and finally after mechanical stirring plus chemical reduction of the surface tension. The measured blood loss, based on haemoglobin content of the fluid, was significantly greater after the third procedure than when only manual stirring was used. To establish the clinical consequences of this observation, measurements of blood loss were based on sets of ten determinations of haemoglobin in irrigation fluid in two groups of operations. In one group the postoperative stirring of the fluid was manual and the results were scattered within a wide range, indicating unreliability of blood loss determinations based on a single sample. In the other group, mechanical and chemical preventive measures were used against formation of blood clots and the measurements showed little variation. It is concluded that complete lysis of the blood in irrigation fluid is necessary for reliable calculation of blood loss. An adequate method to achieve this objective is indicated.