Seven groups (2596 subjects) were vaccinated with a human live anthrax vaccine (HLAV) by three different routes (scarification, subcutaneous and aerosol). The vaccinees were tested for anthrax cell-mediated immunity using the "Anthraxin" skin test at 7, 15, 30, 90, 180 and 365 days following vaccination. The kinetic pattern obtained from all groups, shows a significant, five-phased curve: phase I (2-6 days post-vaccination) shows a slow increase in positive Anthraxin skin reactions. Phase II (7-15 days post-vaccination) shows an exponential rise to a maximum at day 15. Phase III (16-30 days post-vaccination) shows a decrease to day 30. Phase IV (31-90 days post-vaccination) leads to a relative restoration of the positive skin reactions. During phase V (91-365 days post-vaccination) there is a continuous decrease in positive Anthraxin skin reactions. The loss of the skin test reaction on day 30 is a characteristic feature of post vaccination anthrax cell-mediated immunity. It may be due to a blockade of macrophages by lethal anthrax toxin released by the multiplying vaccine strain. Epidemiological observations of HLAV protective rates correlate with the phases of the skin reaction kinetics.