The avidin-biotin immunohistochemical (ABC) method was applied to the diagnosis of alveolar hydatid disease in patients from the hyperendemic region of western Alaska. The test was specific for larval cestodes of the genus Echinococcus, and exhibited high sensitivity. A negative reaction was obtained in all cases in which spontaneous death of the larval cestode had occurred. Results were inconsistent in sections of lesions from patients who had received long-term therapy with mebendazole, because of the persistence of antigens for a considerable time following the death of the parasite.