comparison of the in vitro bactericidal activity of normal and activated macrophages indicated that killing of the virulent strain of opsonized Salmonella typhimurium C5sr by normal macrophages was enhanced by the presence of fresh serum in the culture medium. In contrast, there was no such requirement for the bactericidal activity of activated macrophages. The enhanced killing of the bacteria by activated macrophages occurred both in the presence and absence of serum, and this was not accounted for by the existence of macrophagebound immunoglobulin. The factors in normal rabbit serum which potentiated the killing of bacteria by normal macrophages were suspected to be components of the complement system. Heat-stable factors (immunoglobulins) in normal rabbit serum were effective only if they were surface bound immunoglobulins.