The comparative sensitivity and specificity of four technical variants of the indirect hemagglutination test (IHA) for hydatidosis with tannic acid, glutaraldehyde, benzidine and formol treated cells, the same pool of hydatid cyst fluid and sera from hydatid and non-hydatid persons was studied. The number of reactors in each group of sera and the degree of reactivity of each serum sample varied with the type of IHA test. The sensitivity and specificity of each technique was related to the criterion on which IHA test positivity was based and to the group of sera examined. Of the four techniques, that employing tannic acid was considered as the choice IHA test for hydatid immunodiagnosis. The findings are discussed in terms of their implications to the use of a standard serum of known IHA test titer for reference purposes and to the significance of IHA test results for diagnostic and seroepidemiological studies.