The elaboration in vitro of protective antigen of Bacillus anthracis in chemically defined media has been described in previous reports (Wright etal., 1954a; Puziss and Wright, 1954). The alumprecipitated antigen was effective in immunization of laboratory and domestic animals and well tolerated in man (Wright et al., 1954b; Schlingman et al., 1956; Jackson et al., 1957). The antigen was elabor...