The mechanism by which interference with the biosynthesis of bacterial cell wall causes death and lysis of bacteria appears more complex than originally thought. In an earlier model of the mode of action of beta-lactams, it was assumed that, in the presence of the antibiotics, bacteria synthesize a mechanically weak (poorly cross-linked) cell wall that ruptured under the osmotic-mechanical pressure of the normally growing cytoplasmic mass. However, recent findings suggest a much more complex picture. Lysis and, in at least some bacteria, loss of viability as well, seem to be catalyzed by autolytic enzymes (murein hydrolases), the destructive activity of which is triggered in the beta-lactam-treated bacterium via a poorly understood mechanism. Furthermore, different species of bacteria respond quite differently to treatment with the same beta-lactam: some bacteria are both killed and lysed, others only lose viability, whereas still other species respond mainly by a reversible inhibition of growth (beta-lactam-tolerant bacteria). In addition, structurally different beta-lactams may cause quite different biochemical, morphological, and antibacterial effects, even within the same bacterial species. It is conceivable, therefore, that there is more than one mechanism for loss of viability and/or lysis. Most of the bacteria examined so far contain a number (four to eight) of different penicillin-binding proteins. Genetic and physiological evidence obtained in E. coli indicate that these proteins play essential roles in a variety of physiological functions, such as maintenance of structural integrity, shape, and cell division. Pneumococci with a suppressed autolytic system are resistant to he lytic (and, partially at least, to the bactericidal) effect of beta-lactams. Interference with cell wall synthesis seems to trigger autolysin activity by upsetting the cellular control of autolytic enzyme. It is suggested that the irreversible antimicrobial effect of beta-lactams may have an indirect mechanism in other bacteria as well.