Heat shock protein 70 (hsp70) was chosen as a model antigen with which to investigate autoantibody production in humans with cystic hydatid disease. Levels of specific serum antibody were assessed in the sera of patients with surgically confirmed infection with E. granulosus and sera from non-infected controls. Antigens used were human hsp70, obtained from K562 cells grown in culture, and E. granulosus hsp70 obtained by expression of the full length protein in Escherichia coli following cloning of the associated mRNA. Antibody reactivity to human hsp70 was detected in the sera of only a small proportion of hydatid patients (10%) as well as a similar proportion of sera from age matched controls. Specific antibodies reactive with E. granulosus hsp70 were detected in 60% of hydatid patients, although some samples (21%) from healthy controls also reacted with E. granulosus hsp70, the level of reactivity was significantly higher in hydatid patients. This report identifies E. granulosus hsp70 as an immunogen during human hydatid infection but, despite its having a predicted 81% protein sequence homology with human hsp70, it does not appear to induce autoimmune reactivity against the homologous human protein.