The result of penicillin prophylaxis of experimental inhalation anthrax in the Rhesus monkey has been shown to be dependent upon the time of its initiation. If begun too early, it is unsuccessful. It may not be too long delayed, or the infection will have passed the point of no return. A brief, critical time period exists, during which successful prophylaxis may be initiated. This favourable outcome is attributed to the elaboration in vivo of sufficient antigen or antigens to stimulate an adequate immune response, prior to the initiation of antibiosis.