We describe a method for neonatal screening for biotinidase (EC 3.5.1.12) deficiency. Biotinidase activity is assessed colorimetrically from dried samples of whole blood spotted on the same filter papers as used in the neonatal screening for phenylketonuria. After the reaction, samples from normal infants are characteristically purple, whereas those from affected individuals are straw-colored. To confirm the deficiency, the enzyme is quantitatively assayed in additional blood spots or serum. A pilot study has been initiated with samples obtained by the Commonwealth of Virginia for phenylketonuria testing.