A new method is described for rapid and economical screening of large numbers of hospital nursery infants for elevation in blood phenylalanine associated with phenylketonuria. Results are presented for 682 infants, 96% of whom were 4 days of age. None of the blood phenylalanine values were found to be as high as 4 mg/100 ml, and only 8% were above 2 mg/100 ml. These values appear to be in agreement with values obtained by other methods, and indicate that a very low rate of "false-positives" will be encountered during screening of the 10,000 or more infants that may be necessary to detect a case of phenylketonuria. It is recommended that any result of 6 mg/100 ml or above be considered positive, and require confirmation by phenylalanine determination of a second blood specimen.

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