Biochemical changes which occur in a smooth strain of Escherichia coli were investigated with hyperimmune rabbit serum as a source of antibody and fresh normal guinea pig serum as a source of complement. Ribonucleic acid synthesis, as shown by incorporation of (3)H-uridine, was decreased as early as 5 min, and deoxyribonucleic acid synthesis, shown by incorporation of (3)H-thymidine, was decreased after 15 min of reaction. Incorporation of glycerol-2-(3)H into membrane lipid ceased after 25 to 30 min, probably as a result of functional or physical disruption of the membrane, or both. Permeability control (as indicated by loss of (3)H-uridine-labeled compounds and by decrease in optical density) and protein synthesis were subsequently shown to be affected after 30 min. The metabolic state of the bacteria was found to be important in determining the outcome of the reaction. This was shown by the influence of the type of medium on the reaction. A complex nutrient medium decreased susceptibility as compared with a simple medium. The energy sources glycerol and acetate also decreased susceptibility. It is postulated that the ability of the cell to prevent or repair damage to the cell membrane may be involved. Therefore, metabolic conditions which allow retention of vital processes associated with the cytoplasmic membrane and cell surface will mitigate the bactericidal effect. Such conditions may occur in vivo during bactericidal reactions.