Ether-water (EW) extraction of Pasteurella tularensis produced better antigens than five other chemical procedures. EW extracts produced from stationary-phase, liquid-grown, saline suspensions of strain SCHU S4 cells regularly induced agglutinin and precipitin formation in rabbits. Mice, guinea pigs, and monkeys also responded to EW extracts but with lower antibody levels. The use of strains of lower virulence, acetone-dried cells, organisms grown on a solid medium, and abbreviated extraction conditions all resulted in extracts with a diminished antigenicity, but logarithmic-phase and stationary-phase cells yielded equivalent EW extracts. The use of adjuvant, hyperimmunization, and large doses of antigen increased the precipitin responses of rabbits without appreciably altering the agglutinin response. By the appropriate combination of centrifugal fractionation of EW extracts, use of adjuvant, and vaccination schedule, rabbit antisera with either predominantly agglutinating or precipitating activities were obtained.