397 sera were tested by passive hemaglutination and weinberg reactions to diagnose hydatid cysts. 5 of 211 sera (2.53%) gave positive and 206 sera (97.63%) gave negative results by passive hemagglutination test, whereas in weinberg reaction 204 of them (96.68%) were negative and 7 (3.32%) anticomplementary. 13 sera (14.13%) from another batch of 92 sera gave negative results and 79 of them (85.87%) remained negative by passive hemagglutination test, when 6 of them (6.52%) were positive and 96 (93.48%) negative by weinberg reaction. 56 (91.80%) of 61 sera taken from patients operated on for hydatid cysts gave positive and 5 (8.20%) of them negative by passive hemagglutination test. 29 (47.54%) were positive, and 32 (52.46%) negative by weinberg reaction. In another batch of 15 sera taken from patients, 3 were positive and 12 were negative by passive hemagglutination, 2 were positive and 13 were negative by weinberg test. When the two tests are compared, passive hemagglutination test gave better results both qualitatively and quantitatively than weinberg so that it may be preferred in laboratories having improved facilities.