In 11 infants (birth weight greater than 1800 g) fed a new type of humanized formula with a low phosphorus (P) content (calcium (Ca) 11 mmol/l, P 7.2 mmol/l, sodium (Na) 8.3 mmol/l) biochemical parameters of blood, serum and urine were determined. In nine boys Ca and P balances were evaluated also. Renal net acid excretion was low (0.85 mmol/kg/day). Mean concentrations of P and Ca in urine were 0.34 mmol/kg/day (10.5 mg/kg/day) and 0.1 mmol/kg/day (4 mg/kg/day), respectively. In four infants, Ca concentration in urine was, however, greater than 0.15 mmol/kg/day) (6 mg/kg/day). In infants with birth weights greater than 1800 g fed the new, low-P formula, the low renal net acid excretion, the normal P and the high Ca concentrations in urine were comparable to term infants fed human milk. The high calciuria in several infants may be normal physiologic values. However, it remains to be established that the urinary solubility product of infants fed the new, low-P formula is in the same range as those for infants fed human milk. Unexpectedly, low urinary Na excretion (0.26 mmol/kg/day) and increased urinary excretion of aldosterone-18-glucuronide indicated biochemical evidence of Na deficiency secondary to low Na intake and a high weight gain. If the new, low-P formula is to be fed to infants with a birth weight as low as 1800 g. Na content should be higher than in mature human milk because of the often relatively higher weight gain.