The antibacterial property of serum, first reported by Nuttall in 1888, has attracted many investigators and an extensive literature has accumulated. The sera of most mammals tested have been found to possess bactericidal activity, but a notable exception is the mouse, whose serum, apparently through lack of complement, is unable to kill any of a number of bacterial strains which are sensitive to other sera (Marcus, Esplin & Donaldson, 1954). Many types of bacteria are sensitive to serum killing, although definite lysis does not invariably occur. Most workers, including ourselves, have been con- cerned primarily with killing, as measured by failure to produce colonies on a culture plate. Table 1. Inactivation of the bactericidal power of human serum by inactivation of the components of complement