This study illustrates that the value of both alkali denaturation methods is somewhat limited. At high percentages of fetal hemoglobin, the results obtained with the method of Singer and coworkers, are about 10 per cent too low. The Brinkman and Jonxis method is not accurate when the percentage of fetal hemoglobin is lower than 10 per cent. The alkali resistant fraction found with the method of Singer and associates in adult blood, is a mixture containing fetal hemoglobin. The presence of COHb-A will interfere with the determination of fetal hemoglobin, especially when the optional alkali denaturation method is used.