The aim of the present study was to determine the concentration of plasma prekallikrein in healthy infants during the first year of life. Prekallikrein levels evaluated by a chromogenic substrate assay were low in the first 15 days of life and increased progressively from the third week of life, reaching normal adult values after the third month of life. The low levels of prekallikrein during the neonatal age reflect the impaired ability of liver cells to synthesize proteins.