In the presence of 20% sucrose, exposure of S. dysenteriae and Escherichia coli to usually bactericidal concentrations of "normal" human sera resulted in survival of the bacteria and formation of wall-deficient spheroplasts. These "serum spheroplasts" differ from "penicillin spheroplasts" in several respects, including their susceptibility to osmotic shock. The relation of these observations to certain aspects of the phenomenon of bacterial lysis by serum is discussed.