New penicillin antibiotics are disclosed which are effective antibacterial agents and which belong to the class of .alpha.-(heterocyclic acylamino) penicillins and their salts and esters together with pharmaceutical compositions containing the same and their method of administration. The new compounds are characterized by having an amino substituent on the heterocyclic ring which is linked through an acylamino group to the .alpha.-carbon atom of the penicillin side chain. Procedure for preparing the new penicillins is described as well as the antibacterial activity of exemplary compounds against typical microorganisms.