Cell walls obtained from tomato suspension cultures were treated at pH1 for 1h at 100 degrees C to remove arabinose oligosaccharide substituents from the hydroxyproline residues of extensin. Tryptic attack of these acid-stripped walls yielded glycopeptides containing galactose. When one of these glycopeptides (designated S(2)A(6); sequence NH(2)-Ser-Hyp-Hyp-Hyp-Hyp-Ser-Hyp-Lys-CO(2)H) was treated with (a) NaOH-NaBH(4) or (b) NaOH-Na(2)SO(3) some of the serine was converted into (a) alanine or (b) cysteic acid, and the peptide lost galactose. Maleylation or 3-carboxypropionylation of N-terminal serine was necessary for conversion of this residue and for complete loss of galactose. These results indicate that a single galactose residue is attached O-glycosidically to each of the two serine residues. Hydrazinolysis of peptide S(2)A(6) or of isolated cell walls also led to destruction of serine. In control experiments non-glycosylated serine was not destroyed during hydrazinolysis. Thus the galactosylserine linkage is sensitive to N(2)H(4).