The nature of the different molecular-mass forms of phenylalanine hydroxylase in rat livers was examined by immunoprecipitation of the enzyme from crude liver extracts that had been radiolabelled by reductive methylation. The two forms of the enzyme were resolved by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and detected by fluorography. Segregation of the two forms of the enzyme was demonstrated in Sprague-Dawley rats, as would be expected if the two forms were products of allelic genes. In addition, hooded and albino Wistar rat livers contained only the slower-migrating form and Lewis rat livers contained only the faster-migrating form, and hence we suggest that the forms be referred to as W (for Wistar) and L (for Lewis). Peptide mapping showed that the W and L forms are closely related, and the difference between them appears to reside at one or other end of the polypeptide chain. The kidney contained the same forms as the liver in one-tenth the quantity, providing further evidence that the same phenylalanine hydroxylase gene is expressed in liver and kidney.