Estimation of plasma phenylalanine and tyrosine by an abridged (30 min) column chromatography procedure was assessed. Plasma phenylalanine and tyrosine concentrations, determined in 112 phenylketonuric obligate heterozygotes and in 88 normal controls, did not differ significantly from other reported series in which the data were obtained by column chromatography. All these series were combined and analyzed statistically. Phenylketonuric heterozygotes can be detected with a high degree of discrimination by the ratio of phenylalanine to tyrosine in the plasma of the fasting subject, in combination with the plasma phenylalanine concentration.