Abstract: P-Trimethylsilyloxy-D-(-)-phenylglycyloxysuccinimide or -phthalimide, process of preparing p-trimethylsilyloxy-D-(-)-phenylglycyloxysuccinimide or -phthalimide by reacting D-(-)-p-trimethylsilyloxyphenylglycin-N-carboxyanhydride with a N-hydroxyimide, and process for preparing 6-(p-hydroxyphenylglycylamido)-penam or 7-(p-hydroxyphenylglycylamido)-cephem compounds by reacting p-trimethylsilyloxy-D-(-)-phenylglycyloxysuccinimide or -phthalimide with a phosphite amide of an ester of 6-aminopenicillanic acid or of an ester of a 7-amino-3-cephem-4-carboxylic acid in the presence of the hydrochloride of a weak amine.