Penicillin sulfoxide esters are reacted with an isocyanate to produce the corresponding (substituted)-2-carbamoyloxymethylpenam, the corresponding (substituted)-3-carbamoyloxycepham or the corresponding 3-methylcephem. The 6- or 7-side-chain of these products may be cleaved to give the corresponding 6-amino (penams) or 7-amino (cephams and cephems) compounds, and the latter may be reacylated to produce different 6-acyl-2-carbamoyloxymethyl penams, 7-acyl-3-carbamoyloxy cephams and 7-acyl-3-methylcephems. The substituent groups may be removed from the (substituted)-2-carbamoyloxypenams or the (substituted)-3-carbamoyloxycephams to give the corresponding free 2-carbamoyloxymethylpenams or 3-carbamoyloxycephams, respectively.