In 1934 Folling described phenylpyruvic oligophrenia and the characteristic color reaction of the urine with ferric chloride. 1 Since then the disease has been related to a hereditary deficiency of the enzyme which converts phenylalanine to tyrosine, and lately it has been shown that the earlier in life a diet low in phenylalanine is instituted, the better the chances for ameliorating the convulsive, behavioral, and intellectual disturbances of this disorder. 2 Hence the ferric chloride test, already widely used in the diagnostic evaluation of mental deficiency, has assumed practical importance. Recently the usefulness of a simple and reliable paper strip method (Phenistix reagent strips) of detecting phenylketonuria has been reported. 3 The present note deals with the use of an improved form of Phenistix. The test material consists of a stiff strip of cellulose impregnated with ferric and magnesium ions and cyclohexylsulfamic acid. The ferric ions react with phenylpyruvate to