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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).

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In the past, it was believed that nearly all patients afflicted with this disorder were mentally retarded. In a survey covering all published cases, Jervis 1 found that only three out of 330 phenylketonuric children (0.9%) had an intelligence quotient above 70. In 1960, Knox2 reported that there were 20 known untreated patients with phenylketonuria having "high grade" intelligence. Of these, eight patients had IQs between 60 and 69; six patients had IQs between 70 and 79; three patients had IQs between 80 and 89; and three patients had IQs over 90. More recently, Knox3 has expressed the view that no single factor can account for the relatively mild intellectual defect in these "atypical" phenylketonuric children. He believes that some of these have a biochemical defect which is often as severe as in the usual patient with phenylketonuria. But there appears to be a second group of patients with lower plasma phenylalanine levels; a few also have mildly affected relatives.The widespread screening for phenyl¬ ketonuria among newborn infants has led to the recognition that not all instances of hyperphenylalanemia are caused by

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