The antigenic composition of the live vaccine strain of Francisella tularensis was investigated. Ether-water extracts, water-soluble material from freeze-pressed bacteria and detergent-eluted material from bacterial envelope were allowed to react in immunodiffusion and immunoelectrophoresis with rabbit antiserum against disintegrated bacteria of the vaccine strain. Ten antigenic factors were distinguished in an ether extract. When the extract was precipitated with ammonium sulphate 15 antigenic factors were distinguished in the precipitate and 14 factors in an ethanol precipitate of the supernatant fluid. In the water extract of freeze-pressed material 17 antigenic factors were found. Comparative immunoelectrophoresis of all these fractions demonstrated a minimum of 20 antigenic factors. When envelope material of the vaccine strain was treated with Triton X-100, three more antigenic factors were found to be solubilized. Thus, a total of 23 antigenic factors were distinguished in the extracts. There was a wide variation in heat and trypsin sensitivity between the antigenic factors. A few of the factors had esterase or alkaline phosphatase activity, whereas acid phosphatase, beta-glucuronidase or peroxidase activities were not found in any of the factors.