Extracts of whole body and of venom sac of yellow jackets (Vespula pennsylvanicus) were fractionated by DEAE-cellulose chromatography. Ten fractions were separated from whole body, and seven fractions from venom sac preparations. Only fraction 1 of venom sac was active in direct skin testing. While several fractions of whole body were active by skin testing, only one of them, fraction 1, was significantly active in a patient who was sensitive to yellow jacket only. Other fractions, therefore, reflected the cross-reaction between the different insects. The antigenic heterogeneity of each fraction was analyzed by appropriate rabbit antisera to yellow jacket extracts. Eight of the ten body fractions gave precipitation, as did three of the seven venom sac fractions. They variously gave one or two lines of precipitation. In addition, fractions 6, 7, and 8 of whole body showed, by testing with rabbit antisera, common reactions with wasp and bee. An analysis of the studies comparing the findings of bee, wasp, and yellow jacket extracts and fractions with each other is presented at the end of this paper.