Although ampicillin is often only bacteriostatic for Listeria monocytogenes in vitro, serum from ampicillin-treated patients was bactericidal. The bactericidal effect of serum was partly removed by bentonite, Seitz-filtration, and addition of FeCl3, suggesting it is mediated by lysozyme and beta-lysin. Partly purified human beta-lysin plus purified human lysozyme or either protein plus ampicillin were bactericidal for L. monocytogenes. Hen egg white lysozyme was not active. Lysozyme and beta-lysin were not synergistic with tetracycline, trimethoprim/sulfamethoxazole, or chloramphenicol. Thus, lysozyme and beta-lysin may play a role in the natural resistance to L. monocytogenes, and these serum proteins could contribute to the effectiveness of ampicillin in vivo.