This invention relates to novel 7-acylaminocephalosporanic acid derivatives of high antimicrobial activity, to processes for their preparation, and to pharmaceutical compositions comprising said derivatives, said derivatives being of the formula: ##STR1## wherein R.sup.1 is amino or protected amino,R.sup.2 is dialkanoyloxy(lower)alkyl; alkyl having one or more substituents selected from the group consisting of hydroxy, protected hydroxy, alkoxy, carboxy, protected carboxy, cycloalkylcarbonyloxy and heterocyclic group; lower alkoxycarbonyloxy(lower)alkyl; azido(lower)alkoxycarbonyloxy(lower)alkyl; aroyloxy(lower)alkyl; higher alkanoyloxy(lower)alkyl; phthalidyl; or phthalidylidene(lower)alkyl,R.sup.3 is lower alkyl,Y is thio (--S--) or sulfinyl (--S--), and the dotted line represents 2- or 3-cephem nuclei.