A FIELD TRIAL OF AN INHIBITION ASSAY METHOD FOR SCREENING FOR PHENYLKETONURIA (PKU) TESTED MORE THAN 400,000 NEWBORN INFANTS PRIOR TO DISCHARGE FROM THE HOSPTIAL. IN ALL, 39 CASES WERE FOUND, A HIGHER INCIDENCE THAN HAD PREVIOUSLY BEEN EXPECTED. THE PRACTICALITY OF THE INHIBITION ASSAY METHOD WAS ALSO DEMONSTRATED. THE REPORT DETAILS THE TRIAL'S MATERIALS AND METHODS, INCLUDING ORGANIZATION, SPECIMEN COLLECTING MATERIALS, LABORATORY MATERIALS, LABORATORY TRAINING, AND FOLLOWUP AND CONFIRMATORY PROCEDURES. RESULTS ARE SUMMARIZED FOR BOTH BLOOD AND URINE TESTS. DETECTION OF "OCCULT PKU," OLDER SIBLINGS, PKU IN A NEGRO, AND "MATERNAL PKU" ARE REPORTED. THE DISCUSSION TREATS PRINCIPAL CONCLUSIONS, BLOOD ELEVATIONS IN NON-PKU INFANTS, LABORATORY CONFIRMATION OF PKU, THE PROBLEM OF EARLY HOSPITAL DISCHARGE, THE MASSACHUSETTS PROGRAM, SCREENING OF INSTITUTIONALIZED POPULATIONS, AND OTHER INHIBITION ASSAYS. A SUMMARY AND RECOMMENDATIONS ARE INCLUDED. TABLES PRESENT (1) RESULTS OF BLOOD AND URINE TESTS FOR DETECTION OF PKU IN THE HOSPITAL AND (2) PKU TRIAL PROGRAM CASE FINDING INFORMATION. A BIBLIOGRAPHY LISTS 21 ITEMS. FIVE REFERENCES ARE INCLUDED IN THE DOCUMENT AS EXHIBITS--AN ARTICLE BY ROBERT GUTHRIE AND ADA SUSI ON "A SIMPLE PHENYLALANINE METHOD FOR DETECTING PHENYLKETONURIA IN LARGE POPULATIONS OF NEWBORN INFANTS," A PROPOSAL SUBMITTED BY GUTHRIE TO CHILDREN'S BUREAU ON "PHENYLKETONURIA DETECTION IN A YOUNG INFANT AS A ROUTINE HOSPITAL PROCEDURE, A TRIAL OF A PHENYLALANINE SCREENING METHOD IN 400,000 INFANTS," A LIST BY GUTHRIE OF "SUGGESTIONS CONCERNING COLLECTION OF BLOOD AND URINE SPECIMENS," AN UNPUBLISHED MANUSCRIPT BY GUTHRIE AND SUSI ON "LABORATORY CONFIRMATION OF PHENYLKETONURIA," AND A MEMORANDUM FROM GUTHRIE ON "DETECTION OF 'MATERNAL PHENYLKETONURIA.'" ALSO INCLUDED IS A SUPPLEMENT ON "PROCEDURES USED IN MASSACHUSETTS PKU SCREENING PROGRAM" BY ROBERT A. MACCREADY, JOANNE WOOD, VALERIE KAROLKEWICZ, AND WALTER T. BROWN. THIS DOCUMENT IS AVAILABLE FROM THE SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, D.C. 20402, FOR $.30. (JD)