Bovine and human hydatid antigens collected from hepatic cysts and characterized by SDS-PAGE immunoblotting show similar patterns. The bovine hydatid antigen has been used to develop a simple and fast in vitro diagnostic assay for human hydatidosis. This method, named HA-DIA (hydatid antigen dot immunobinding assay), consists of incubation of a serum sample with a textile colloidal dye (pink) and a nitrocellulose stick to which the hydatid antigen has been bound. The presence of parasite-specific antibodies leads to dyeing of the stick reactive area, and a coloured spot appears. HA-DIA sensitivity and specificity have been studied in comparison with RAST-IgE and ELISA-IgG by testing 17 sera of patients with hydatid disease and 36 control sera from patients affected with other parasitic and non-parasitic diseases. HA-DIA showed positive results in all the patients' sera and in none of the control sera. Correlation with ELISA--IgG and RAST-IgE was significant. HA-DIA has been demonstrated to be of good predictive value, allowing a speedy diagnosis of hydatid disease. In view of its simplicity, not requiring any laboratory instruments, it is particularly suitable for large-scale field screening.