Biochemical investigations of the mode of action of beta-lactam antibiotics have focused on the interaction of these drugs with sensitive enzymes and penicillin-binding proteins in vitro and on the correlation of these data with physiological responses to the drugs. The classical response is inhibition of growth and cell death followed by lysis; however, the bacteriostatic response to penicillins, which is seen in certain bacterial species such as Streptococcus mutans and in mutants of species such as Streptococcus pneumoniae, is also described. The biochemical data remain consistent with the acyl-D-alanyl-D-alanine donor substrate analogue theory, but multiple targets with differential sensitivity to different beta-lactam antibiotics exist. The relationship of these targets to penicillin-binding proteins and their possible functions in cell growth and division are discussed.