[[[(2,4-Dioxo-1-imidazolidinyl)amino]carbonyl]amino]-acetylpenicillin derivatives of the formula ##STR1## wherein R is hydrogen, lower alkyl, phenyl-lower alkyl, tri(lower alkyl)silyl, a salt forming ion, or the group ##STR2## R.sub.1 is hydrogen or methoxy; R.sub.2, R.sub.3 and R.sub.5 each are independently selected from hydrogen and lower alkyl; R.sub.4 is hydrogen, lower alkyl, cycloalkyl, cycloalkenyl, cycloalkadienyl, phenyl, phenyl-lower alkyl, substituted phenyl, substituted phenyl-lower alkyl, or certain heterocyclic groups; R.sub.6 is lower alkyl; are disclosed. These compounds are useful as antibacterial agents.