6-Acylamino-penam-3-carboxylic acids and 7-acylamino-3-cephem-4-carboxylic acids in which the acyl group has the formula ##STR1## in which R.sub.1 is hydrogen, R.sub.2 is optionally substituted phenyl, thienyl or furyl or R.sub.1 and R.sub.2 together are optionally substituted cycloalkyl, n and m independently of one another represent 0 or 1 and B represents an optionally substituted 2,6-dioxo- or 2,6-dithioxo-1,2,3,6-tetrahydro-pyrimidyl, 2-thioxo-6-oxo-1,2,3,6-tetrahydropyrimidyl, 2,6-dihydroxy- or 2,6-dimercapto-pyrimidyl, 2-mercapto-6-hydroxy-pyrimidyl, 2,6-diaminopyrimidyl, 2-amino-6-hydroxy-pyrimidyl or 2,6-dihalogenopyrimidyl radical, these radicals being bonded in the 4- or 5-position.