Abstract In order to develop a logical method of purifying antigenically active fractions of Ascaris lumbricoides, we devised a hemagglutination-inhibition technique. This test measures the ability of a particular fraction to inhibit the activity of a standard antiserum in the hemagglutination test. A homogenate of adult A. lumbricoides obtained by treatment of infected persons was extracted with phosphate-buffered saline solution. The resulting extract was subjected to treatment with 95% ethanol, saturated ammonium sulfate, and chloroform-normal butanol; the resulting fractions were compared. Attempted chromatographic separation of the fractions was unsuccessful. The effect of digestion with papain and amylase on antigenic activity was also noted. The hemagglutination-inhibition test identified three active precipitates of A. lumbricoides obtained with ethanol with or without papain digestion and with ammonium sulfate. Papain digestion destroyed the ability of the fraction to sensitize erythrocytes but not its activity in the hemagglutination-inhibition test. Amylase digestion destroyed antigenic activity. Consequently, we concluded that antigenicity of the polysaccharide-protein moiety obtained by ethanol precipitation resides in the polysaccharide portion, while the protein portion is necessary to allow the antigen to coat the erythrocytes.