A process for preparing a .beta.-lactam derivative of formula (2) and/or the corresponding enol tautomer, which includes oxidizing an alkenyl-substituted .beta.-lactam derivative of formula (1) in the presence of a ruthenium catalyst or a rhenium catalyst using a periodic acid: ##STR1## wherein R.sup.1 is a hydrogen atom, a C.sub.1-4 alkyl group or a C.sub.1-4 haloalkyl group, R.sup.2 is a hydrogen atom, a C.sub.1-4 alkyl group or a phenyl group which may have a substituent, A is a group selected from ##STR2## wherein R.sup.3 is an amino group or a protected amino group, and R.sup.4 -R.sup.8 are as defined in the application.