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Protein utilization in very low birth weight infants fed the formula MANASAN or human milk protein. The influence of protein utilization was studied in 19 very low birth weight infants appropriate for gestational age between the 5th and 6th week of postnatal life. 8 of the infants were fed with the formula MANASAN (protein: cow's milk casein) and 11 were fed with similar quantities of human milk (HM) protein (HM fortified with 6g freeze-dried HM/100 ml). On two following days the protein intake and the nitrogen excretion in urine and stools were measured and the nitrogen balance was calculated. In the urine the excretions of alpha-amino-nitrogen, urea, ammonium and sulfuric acid were also estimated. On one of both study days the concentrations of alpha-amino-nitrogen and urea were measured in the preprandially obtained serum. The weight gain was calculated as mean of two weeks ending with the last day of study. Despite the similar protein intakes and the sufficient energy intakes in both study groups the serum concentration of alpha-amino-nitrogen and the renal excretions of alpha-amino-nitrogen, ammonium and sulfuric acid were significantly higher in the groups fed MANASAN than in the groups fed HM protein. The excretion of ammonium in the urine was significantly correlated to the excretion of sulfuric acid. The weight gain and the nitrogen balance were significantly lower in the infants fed MANASAN in comparison to the infants fed HM protein.(ABSTRACT TRUNCATED AT 250 WORDS)