Summary By the employment of complement fixation and indirect hemagglutination tests 50 serum samples obtained from patients with hydatid disease and other conditions were tested. Of the 16 serum samples collected from hydatid disease patients positive results were obtained in 13 (81.25%) in indirect hemagglutination tests and in 11 (68.75%) in complement fixation tests. Of the 34 serum samples obtained from patients with other than hydatid disease the number of positive reactors was zero in indirect hemagglutination tests, and 2 (5.88%) in complement fixation tests. The results of the limited number of determinations indicate that indirect hemagglutination is a more sensitive and more specific serologic diagnostic procedure for hydatid disease than is complement fixation. Furthermore, it is easier to run than the complement fixation test and is less expensive.