Fat excretion and absorption in 38 healthy S-f-D born infants fed on 4 kinds of diet, differentiated with regard to fat quantity and quality as well as protein quality, were examined by means of three 24 hour fat balances. Higher fat intake, higher fat excretion in feces and lower index of its absorption, typical of these infants, were observed. Differences in relation to the infants born with normal body-weight were particularly distinct in the first six months of life. It was noted that the quantity of fat excretion in feces was influenced by the degree of fetal malnutrition. An advantageous influence of fat modification, which consisted in partial replacement of cow-milk fat with vegetable oil (soya oil) on fat excretion and absorption, was shown. Feeding higher fat formulas--3 g/100 ml and modified protein were also considered advantageous. It was noted that S-f-D infants, even in cases of feeding with modified fat formulas, at the end of the first year of life did not reach such values of the absorption index as infants born with normal body-weight. The conception that the triglyceride structure differing from that in the breast milk is the factor deteriorating cow-milk fat absorption in the infants was suggested and discussed.