SUMMARY: [35S]Cysteine was injected intracisternally into guinea-pigs and 5 h later the soluble proteins of the neural lobe were subjected to electrophoresis on starch and polyacrylamide. In both systems approximately 80% of the total radioactivity was recovered in a single peak with an electrophoretic mobility corresponding to the position of the major protein component. The proteins present in the guinea-pig neural lobe were extracted in 0·1 m-HCl. Fractionation of the protein—neurohypophysial hormone complex on Sephadex G-75 resulted in the isolation of a protein which cross-reacted immunologically with an antiserum raised against porcine neurophysin II. The protein designated a neurophysin contained one molecule of lysine, isoleucine and tyrosine per molecule of protein. It contained no histidine or methionine and its minimum molecular weight was estimated as 8436 by amino acid analysis.