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Mice were inoculated once a week for 3 weeks with 0.2 ml of an antigenic fraction of Echinococcus granulosus scoleces. One week after the last vaccination about 3700 living scoleces/mouse were inoculated into the peritoneum. 5 months later the mice were killed and newly formed cysts in the peritoneum were weighed and counted. Crude antigens from the scoleces and the 0.8 M fraction (the pellet resulting from centrifugation at 16 000 g for 15 min, and the precipitate obtained by treating the supernatant with 0.8 M ammonium sulphate, respectively) afforded significant protection. The other antigens (the supernatant and the precipitate formed with 1.6, 2.5 or 5.0 M ammonium sulphate) did not give consistent immunity. Indirect haemagglutination showed a negative correlation between antibody titre (against the 0.8 M fraction) and the formation of cysts in those mice protected.