Acylaminopenicillanic acids of the general formula ##STR1## in which R.sup.1, R.sup.2 and R.sup.3 represent hydrogen or lower alkyl which may be substituted, and in which the radicals R.sup.1 and R.sup.2 or R.sup.2 and R.sup.3 may form together an alkylene radical which may be substituted, R.sup.4 and R.sup.5 represent hydrogen or lower alkyl, R.sup.6 represents phenyl which may be substituted, a monocyclic aromatic heterocycle which may be substituted, or dihydrophenyl, A represents a benzene or thiophene ring which may be substituted, and X represents oxygen or a single bond, a process for preparing them and pharmaceutical compositions active against bacterial infections containing these compounds.