Antiserum was raised against a synthetic nona-peptide which was predicted to have considerable homology with the unhydroxylated, unglycosylated precursor of cell wall proteins from several plants. The antiserum is able to recognize the major cell wall protein of incubated carrot (Daucus carota) root discs which is produced when the discs are treated with a proline hydroxylase inhibitor and then labeled with radioactive proline. This technique has potential applications in studying cell wall biosynthesis and its regulatory control mechanisms.