A fluorometric assay in microtitre plates for the screening of phenylketonuria was evaluated and adapted to a neonatal screening programme. Using this assay, it is possible to determine quantitatively the phenylalanine concentration in dried blood spots on filter paper. The test exhibited a linear calibration curve with a good slope as well as sufficient precision and accuracy in the statistical analysis. Interference by other amino acids and antibiotics was not observed. Only elevated concentrations of leucine interfered to a small degree. The phenylalanine concentration in dried blood spots of 13 phenylketonuria patients correlated to that in serum. 7381 dried blood samples of newborn infants were tested simultaneously by both the fluorometric and the Guthrie test. The results did not show significant differences. We screened 29,182 newborns using the fluorometric assay and an online data processing programme. The internal repetition rate was 0.64%, the external recall rate 0.15%. False negative results were not observed. In December 1991 the fluorometric method replaced the Guthrie test in our routine programme for phenylketonuria screening, and was introduced as a follow up test for phenylketonuria patients.