The action of 3-1actam antibiotics on sensitive bacteria can be regarded as a two-stage process. In the first stage, the antibiotics bincl to primary receptors, physically identified as membrane-associated penicillin-binding protein (PBP's). These proteins perform central roles in the cell cycle-related, morphogenetic synthesis of cell wall peptidoglycan. Inactivation of PBP's by bound antibiotic has immediate, biochemically definable effects on their function. The second stage encompasses the physiological effects on the sensitive cell initiated by this primary receptor-ligand interaction.