The clinical value of the Coulter S-Plus V three-part leukocyte differential cell count in the preoperative evaluation of 295 patients scheduled for ambulatory surgery was examined. Ninety-five percent of the specimens could be processed by the instrument without further review. Only 11 specimens had abnormal manual differential cell counts. Of these, all but three were detected using a combination of S-Plus V-generated flags and preset laboratory criteria. There were 16 cases with red blood cell morphologic abnormalities, eight of which were not associated with specimens referred for complete manual differential cell count. In none of these 11 unreferred cases was the undetected abnormality of clinical significance relative to the patient's upcoming surgery. The use of the S-Plus V three-part differential in this patient population can result in a significant cost savings without adversely affecting patient care.