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The past two decades has been a period of intensive and fruitful research on the atiology, diagnosis and treatment of the anemias. Although diagnostic methods have been extended and elaborated, research has served also to confirm the fundamental value of accurate determination of the haemoglobin content (Hb) of the blood. Routine estimation of the Hb has revealed the fallacy of assuming that pale people are necessarily anemic; it has also discovered many cases of undoubted anaemia among those who pride themselves on having a "good colour. Although the finding of a low Hb concentration calls for a detailed blood examination, it will be generally agreed that a practitioner who makes a careful clinical study of his patient and considers his findings in conjunction with the result of the Hb estimation will often be in a position to form a valuable opinion regarding the exact type of anemia present. The principle of Sahli's method is simple: "the hemoglobin contained in a known quantity of blood is converted into acid heematin by means of hydrochloric acid. The colour is then compared with a standard tube containing acid haematin of known strength" (Whitby and Britton, I935). The technique is straightforward and can be learned in a few minutes. Despite the simplicity of the method, however, there are numerous sources of error traceable not only to faulty technique but also to unsatisfactory apparatus. The existence of fallacies became apparent while making upwards of five hundred estimations of the Hb in connection with a clinical investigation. The purpose of this article is to make a plea for standardization of both technique and apparatus, and to try to estimate the importance of the various sources of error that are encountered. For reference and discussion the standard method taught to students in this hospital is set out below. Clean Tube. Brush if necessary. Volume of Acid. Exact quantity immaterial; 30 per cent. mark is a convenient level. Obtaining Blood. Sharp, sterile Hagedorn needle. Wipe sweat off lobe of ear with dry swab. Stab skin boldly. Do not squeeze blood out: vigorous percussion of puncture with finger tip promotes blood flow. Do not allow blood to concentrate by drying. Pipette. Narrow bore most satisfactory, e.g. 20 c.mm. mark about 6 cms. from tip. Absolutely dry and clean. Blood free from air bubbles. If blood passes beyond 20 c.mm. mark, carefully reject excess by tapping tip gently against pad of finger. Wipe blood off outside of pipette. Transferring to Acid. With tip of pipette just under surface of acid, gently eject blood in stream to bottom of tube. Fill pipette twice with cold water and add washings to tube. Insert stopper into tube: mix blood and acid by inverting: replace tube in colorimeter for one minute. While waiting, rinse pipette several times with alcohol then with ether, and finally dry by sucking air through. Dilution. As water* is added drop by drop, insert stopper and mix by inverting tube. Do not shake (frothing). Comparing Colours. Source of light-clear sky or bright clo...