Determinations of serum calcium (Ca), phosphorus (P) and calcitonin (CT) were carried out in two groups of full term newborn infants: 80 on bottle feeding (group A) and 80 breast feeding (group B). Samples of blood were drawn at 1, 6, 12, 24, 48, 72, 96 and 168 hours of life. Each newborn was tested no more than twice. No differences between the two groups could be detected before the 72th hour. At that time, the Ca of group A was significantly lower than Ca of group B while the CT and P were significantly higher in the same group A. So that, the P serum level appears to be one of the most important factor regulating the homeostasis of Ca after the first 72 hours of life.