Acarboxyprothrombin (protein induced by vitamin K absence or antagonist-II (PIVKA-II] concentrations in dried blood spots were determined in 19,029 infants at about 1 month of age as an indicator of vitamin K deficiency. We observed 51 cases with raised blood concentrations of PIVKA-II (greater than 4 AU/ml), nine of whom showed very high concentrations (greater than 20 AU/ml). For infants who did not receive vitamin K prophylaxis at birth, the incidence of the PIVKA-II test yielding positive results was significantly higher in those solely breast fed (0.51%) compared with those fed formula milk (0.18%). Among solely breast fed infants, the incidence of a very high result of the PIVKA-II test was 0.14% in those who had not received vitamin K prophylaxis at birth, 0.04% in those who received 2 mg orally, and 0.03% in those who received 2 mg orally plus a further dose of 2-4 mg orally at 7 days. Thus vitamin K prophylaxis at birth did not completely prevent vitamin K deficiency at 1 month. We administered vitamin K therapeutically to all infants whose PIVKA-II test yielded a positive result at 1 month. Only one infant with a positive result developed late neonatal intracranial haemorrhage.