IN 1958, two of us1 obtained, by enzymic degradation of acetyl-chymotrypsin labelled with carbon-142, five peptides containing labelled acetyl-groups. Subsequent sequence studies3 have shown that the largest of these peptides has the amino-acid sequence, Gly.Asp.Ser.Gly.Gly.Pro.Leu, identical with that already found4, by degradation of the di-isopropylphosphoryl-enzyme, to surround the reactive serine ; it may reasonably be concluded that the other four peptides contain the sequences Ser.Gly.Gly.Pro.Leu, Asp.Ser.Gly.Gly, Gly.Asp.Ser.Gly and Ser.Gly.Gly, respectively. © 1960 Nature Publishing Group.