Protein S in the first year of life.

Total protein S, free protein S and C4b binding protein were measured in healthy term infants in order to establish the normal levels of these proteins during the first year of life. Total protein S rose from 36.5% of the adult mean level on day 1 of life to 90% between 6 and 12 months of age. Free protein S was 54.2% of the adult mean on day 1 of life, all values were in the adult range by 2 months and the mean value was no different from the adult at 4 months. This relatively high level of free and presumably active protein S reflects low levels of C4bBP at birth (28.8% of the adult mean) and a slow postnatal rise.