Abstract: A process for producing a 2-cephem or 3-cephem derivative compound of the formula: ##STR1## wherein R.sub.1 represents a substituted or unsubstituted amino radical and R.sub.4 represents hydrogen or a radical selected from the group consisting of carboxy, protected carboxy, ester, acid amide, acid anhydride, acid halide, acid azide and carboxy salt, and the dotted line indicates the alternate bond structure providing 3-cephem or 2-cephem, which comprises:reacting a halogenated derivative selected from the group consisting of a halogenated penam derivative having the formula: ##STR2## a halogenated cepham derivative of the formula: ##STR3## wherein X represents a halogen atom, R.sub.3 represents a radical selected from the group consisting of carboxy, protected carboxy, ester, acid amide, acid anhydride, acid halide, acid azide and carboxy salt, and R.sub.1 is as defined above, or mixtures thereof with a dehydrohalogenoic acid reagent.