Much of the work on immunology of hydatidosis has so far been devoted to the development of suitable methods for serological diagnosis. The precise nature of hydatid antigens and their chemical characterization has still not been worked out, largely because of the complex life-history of the parasite and the difficulties of in vitro cultivation. The most widely used antigen for routine serological testing in hydatidosis caused by Echinococcus granulosus is fluid taken from the cyst. This fluid is, however, a complex mixture of substances and contains several protein and carbohydrate fractions as well as end-products of carbohydrate and protein metabolism. The cyst fluid from different sources is variable in its antigenic properties, and the fluid from sterile cysts is especially lacking in antigenic activity. Antigens from tissue extracts of hydatid cysts appear to have greater specificity. Cyst extracts of E. multilocularis, the cysts of which contain relatively little fluid, have also been used but are poor antigens, and contain measurable amounts of host protein. Antigens prepared from other cestodes and metabolic antigens are also reviewed.Biochemical analysis of Echinococcus antigens covering polysaccharides, proteins, lipids, and blood-group substances is considered, together with the characterization of antigens by electrophoresis, column chromatography and gel-diffusion methods. The problems associated with the standardization of antigens are discussed. Recent data on the character and reactivity of antigens employed in Echinococcus studies are summarized.