Cefprozil monohydrate Impurity: (6R,7R)-7-[(R)-2-Amino-2-(4-hydroxyphenyl)acetamido]-8-oxo-3-[(Z)-prop-1-en-1-yl]-5-thia-1-azabicyclo[4.2.0]oct-3-ene-2-carboxylic acid (Ceprozil delta-3-isomer).