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Spectrophotofluorometric micromethods for the determination of phenylalanine and tyrosine on 25 µl of serum are described. These methods were applied to detect phenylketonuria among homozygotes and heterozygotes and in the newborn population. The data presented agree with those previously reported. (1) Phenylketonurics have markedly elevated serum phenylalanine and lower serum tyrosine than the controls. (2) The heterozygotes have a higher fasting serum phenylalanine and, after a standard oral phenylalanine test, show a higher and more prolonged rise of serum phenylalanine than the controls. There was a lesser increase of serum tyrosine after phenylalanine loading in the heterozygotes than the controls. (3) Premature infants of low birth weight have higher serum phenylalanine and tyrosine levels than normal birth weight infants, presumably due to enzyme immaturity. Simultaneous determinations of serum phenylalanine and tyrosine will differentiate newborn infants who are suspected to be phenylketonurics from the homozygotes (phenylketonurics).