A simple immunoenzyme technique is described in which the enzyme beta-galactosidase of E. coli (Z) is used as a marker. The principle of the test is based on the ability of the cells to bind the antigen. Conditions of the assay using Z both as the antigen and marker are described. As a screening technique, conditions are defined at limited antigen concentrations when binding is not directly proportional to the number of cells present. Hence, drops of blood can be used directly for detection of an immune response without counting the cells. Furthermore, treatment of whole blood with ethanol was shown to (a) increase the binding capacity and (b) allow the storage of specimens for weeks without any loss of activity. With human hydatid fluid (HHF) antigens conjugated to Z as a model, it was possible to detect the immune response of rabbits injected with HHF. The method is simple, requires no sophisticated equipment and can be used in large scale epidemiological surveys.