The prevalence of anæmia among London infants is shown by an investigation extending over the last three years and comprising about 770 cases with 3,100 hæmoglobin estimations. This anæmia is largely nutritional in origin and is due to a deficiency of iron, possibly associated with other factors. It is known that an infant obtains its necessary iron not only from its milk, which is poor in iron, but also from a store in the liver. Our results lead us to suppose that this store may be unduly low in many London infants. The average hæmoglobin percentage in the blood of fairly healthy artificially-fed London infants shows a sharp fall from its high level at birth to about 65 per cent. at 2 to 3 months of age, a rise to about 70 per cent. by 5 to 6 months, and then a steady fall reaching about 65 per cent. at 12 months. The continuous administration of iron, beginning when the infant is under 2 months old, raised the average hæmoglobin percentage to 80 per cent. at 4 months of age and onwards. Infants whose birth-weight is under 6 lb., twins and premature infants are specially in need of iron treatment. The selection of the iron salt is of importance, and iron and ammonium citrate proved effective. It was satisfactorily administered on a large scale by giving it incorporated in a dried milk.No evidence was obtained that want of light is an etiological factor in anæmia in infancy, and artificial light therapy did not cure it. No vitamin deficiency appeared to be involved. The value of liver treatment is still under investigation, but the results so far obtained can be explained on the assumption that liver treatment is efficacious only because it provides a source of iron. The routine administration of an iron salt to artificially-fed infants is advocated on the ground that, by preventing the anæmia from which most of them would otherwise suffer, the level of health of our infant population would be raised.