The Casoni intradermal skin test is a reliable test (87%) for indication of the presence of antibodies to Echinococcus. A reaction to the test can be obtained for years after removal of the cyst or after the parasite has died and has become calcified. Fixation of the complement is equally reliable (93%), but with this method only a living cyst is detected. The reaction to the test becomes negative within 2 to 6 months after removal of the cyst. Antigen can be preserved indefinitely with Merthiolate in a 1:50,000 concentration. The antigenic component of the antigen for fixation of the complement migrates with the gamma globulin fraction on paper electrophoresis, but can be separated from the protein by precipitation with cold acetone-ether mixture. The antibody in the patient''s serum is contained in the gamma globulin fraction. The indirect hemagglutina-tion test appears to be specific for this disease. The serum of patients with hydatid disease forms lines of precipitation in double diffusion agar plates (modified Ouchterlony technic) when tested against cystic fluids of both E. granulosus and E. multilocularis. A single line forms with the former, and double lines or diffuse wide bands with the latter. The serum of white rats injected with multiple doses of hydratid cystic fluid yields positive results to complement-fixation tests, but does not produce lines of precipitation on double diffusion agar plates. The substance in the gamma globulin fraction of the antigen that produces fixation of the complement does not produce a line of precipitation by the double diffusion agar-plate method, but the albumin fraction does. The cystic fluid of Echinococcus is a highly provocative antigen.