Abstract: The present invention relates to a mutant penicillin expandase which comprises an amino acid substitution at one or more residue positions corresponding to those of a wild-type expandase selected from the group consisting of threonine at position 42, isoleucine at position 50, histidine at position 57, threonine at position 67, valine at position 133, threonine at position 143, proline at position 145, glycine at position 148, phenyl alanine at position 152, proline at position 196, alanine at position 240, cysteine at position 281, Serine at position 309, provided that the amino acid substitution at the residue position of cysteine at position 281 is not tyrosine.