Immunological studies have been applied to the immunodiagnosis of human hydatidosis in Turkana and for the specific identification of Echinococcus eggs, particularly in regard to their potential for the assessment of a hydatid control programme in the north-west of the District. A high rate of false negatives has been obtained with sensitive antibody tests for proven hydatid patients in Turkana and presents problems for prevalence estimation in the population. A seroepidemiological study using specific antibody detection indicated a 2.17% seropositive rate, and by using a portable ultrasound 67% of antibody positives were confirmed as having liver/abdominal cysts. An enzyme immunoassay test for specific circulating antigen, often associated with circulating immune complexes, was positive in sera in 75% of hydatid patients who were antibody-negative. The formation of specific circulating immune complexes may contribute directly to the production of a state of false negativity. These studies indicate that by combining tests for antibodies and antigens together with ultrasound, the true prevalence of human hydatidosis in Turkana is at least 5%. Characterization of Echinococcus antigens recognized by human antisera from Turkana and British hydatid patients indicates that both groups recognize the same major protein antigens in cyst fluid and protoscoleces. The results of assaying anti-oncospheral antibodies in people living in the high prevalence area of north-west Turkana compared to people in areas of low prevalence indicates that most of the people in the high prevalence area are probably infected (challenged) with Echinococcus eggs. This observation suggests that there may be a high level of acquired resistance. A species-specific immunofluorescence test using an anti-Echinococcus monoclonal antibody has been developed which can specifically identify oncospheres artificially hatched from Echinococcus eggs. This test will be applied to the problem of identification of Echinococcus eggs in the environment and in dog faeces, with a view to studying the epidemiology of the disease and for use in assessment of the hydatid control programme.