Fecal specimens of 35 parasitologically confirmed cases of giardiasis, 41 acute, gastroenteritis, 23 acute baciUary dysentery, 40 normal persons and 15 jirds experimentally infected with G. lamblia were tested with conterimmunoelectrophoresis (CIE). It was found that 33(94%) out of the 35 giardiasis patients and 14(93%) out of 15 infected jirds showed positive reaction, while fecal specimens of other cases, normal persons, or normal jirds all showed negative reaction. Besides, CIE was performed in 4 giardiasis patients before and after metronidazole treatment. Prior to metronidazole, they were all CIE positive, one day post metronidazole, 3 of them were still CIE positive, and Giardia cysts or trophozoites were also present in their stools. However, from the second day onwards, all of them became CIE negative, while cysts or trophozoites also disappeared from their stools. Apparently, detecting Giardia antigen in fecal specimens with CIE of feces is not only a sensitive tool for detecting current infection, but also a, useful tool for evaluating therapeutic effects in giardiasis.