Fetal and adult hemoglobin pigments have slightly different light absorption coefficients. Blood from newborns therefore gives inaccurate results with a direct spectrophotometric determination of hemoglobin pigments (Radiometer's OSM3), if the absorption coefficients for adult blood are used. We determined an absorption coefficient matrix for hemoglobin pigments in twenty full term newborns' blood. This matrix yielded results accurate to within 0.2% in 40 further newborns, but the accuracy of the results varied with the individual ratio of fetal to total hemoglobin, which was 80 +/- 5% (SD) in the examined samples. The OSM3's inaccuracy with the adult absorption coefficients can be used to directly estimate the ratio of fetal to total hemoglobin in an infant.