Studies with Shigella dysenteriae and Escherichia coli strains revealed enhancement of bacterial susceptibility to normal human serum in vitro in the presence of low concentrations (0.1%) of broth, of glucose, or of certain amino acids known to play a prominent role in bacteria' metabolism. Dipeptides containing these amino acids were more effective than single amino acids. In contrast, metabolic inhibitors, including 5-methyltryptophan, fluorophenylalanine, and non-bactericidal concentrations of chloramphenicol, decreased bacterial susceptibility to serum. Such decrease also was obtained in the presence of, or following exposure to, relatively high concentrations (0.5%) of so-called basic amino acids. The latter effects were reversible by washing when bacteria had been exposed in the absence of nutrients, but they were irreversible following exposure in the presence of nutrients.