Serologic studies on sera from native Mississippi swine with hydatid cysts have shown that Echinococcus antibody can be detected by the indirect hemagglutination test, the complement fixation test, and the precipitin test. The indirect hemagglutination test was more sensitive (76.5% of 64 sera) than the precipitin test (47.2% of 64 sera) or the complement fixation test (3.1% of 64 sera). In testing sera from pigs having cysts of unknown etiology, positive serologic reactions were obtained in 85.7% in the indirect hemagglutination test, in 66.6% in the precipitin test, and in 0.0% in the complement fixation test. Sera from pigs with no evident infection gave positive reactions in the indirect hemagglutination test (22.2%) and were negative in the complement fixation test. The significance of positive serologic tests in sera from pigs with cysts of unknown etiology and from pigs with no evident infection is discussed. Although Echinococcus infection in pigs is of considerable economic importance, since the organs involved must be destroyed, reports concerned with the detection of Echinococcus antibody in infected pigs are uncommon in the literature. Bonelli (1934) was unable to detect precipitins in the serum from swine found to have hydatid disease. Urbain (1936), however, reported that five of seven pigs harboring the infection were positive by the complement fixation test. The pig is the chief host for the larval stage of Echinococcus granulosus in Mississippi (Hutchison, 1960; Hutchison and Bryan, 1960). Since 1957, it has been possible to obtain sera from a number of pigs infected with this parasite. This paper reports the results obtained when these sera were tested for hydatid antibody by means of the indirect hemagglutination (IH) test, the complement fixation (CF) test, and the precipitin (P) test. MATERIALS AND METHODS Pig sera were divided into three groups which included: (1) 64 sera from native Mississippi swine with proven hydatid cysts containing scolices; (2) 35 sera from Mississippi swine having cysts of unknown etiology; and (3) 18 sera from native Mississippi swine which showed no evidence of infection or cystic structures of any kind. The pigs were under 2 years of age and most were under 1 year of age. All were slaughtered at the Jackson Packing Company, Jackson, Mississippi. Received for publication 21 June 1967. * This study was supported in part by Research Grant AI-01237 from NIAID, U. S. Public Health Service. The antigen employed in all tests was pig hydatid cyst fluid from fertile cysts. Before use, the cyst fluid was tested for antigenicity by titration with a serum of high titer. The 50% CF Test of the Serology Research Laboratory, Communicable Disease Center, Atlanta, Georgia, was employed. This procedure is based on the work of Schubert, et al. (1955). The P Test was a simple interfacial ring test using small tubes 50 mm long by 6 mm diameter. Equal volumes of hydatid cyst fluid antigen were layered over dilutions of sera being tested. The tubes were sealed with nail polish to prevent evaporation and were examined periodically up to 72 hr for evidence of precipitation at the interface. Precipitation at the interface of any serum dilution was considered a positive test. The IH Test performed was a modification of that described by Boyden (1951). The antigen was fixed onto tannic acid-treated sheep red blood cells, which had been preserved in Alsever's solution for at least 4 days prior to use. On the day of the test, a portion of the sheep red blood cells was washed three times in phosphate buffered saline (pH 7.2). After the last wash the cells were used in the test if the supernatant was clear. The cells were resuspended in pH 7.2 buffered saline to make a 2.5% suspension. An equal volume of 1:20,000 tannic acid in buffered saline was added and the mixture incubated at 37 C for 10 min. The cells were then washed free of excess tannic acid with buffered saline and resuspended as a 2.5% suspension. One volume of a 1:4 dilution of hydatid cyst fluid antigen in buffer was added to an equal volume of tanned cells and allowed to stand at room temperature for 15 min. The antigen treated tanned cells were washed with 1:250 normal rabbit serum in buffer and resuspended as a 2.5% suspension. Four-fold dilutions of sera were prepared with 1:100 normal rabbit serum starting at a dilution of 1:10. One drop of antigen-treated cells was added to 0.5 ml of diluted serum. The erythrocyte pattern on the