The surveys of the College of American Pathologists indicate a consistent bias between the manual "reference" method for hemoglobin determinations and the Coulter S measurements. The Coulter S hemoglobin values are invariably lower; the difference averages 0.3 g/dl. The commonly used calibration methods for the Coulter appear to be subobtimal. The hemoglobin calibration and quality assurance method proposed by Bull and colleagues is advocated as the presently most acceptable method. Commercial hemoglobin control materials should not be used for hemoglobin calibration.