Isomeric forms of 3-(2-aminoethylthio)-6-(1-hydroxyethyl)-7-oxo-1-azabicyclo[3.2.0]hept-2-en e-2-carboxylic acid (I) ##STR1## are disclosed; also disclosed is a process for their total synthesis; such isomers individually, and as mixtures, are useful as antibiotics.