The use of hybridoma antibodies developed against the sporozoite stage of avian coccidia, coupled with genetic-engineering techniques, has made it possible to begin bird-immunization studies utilizing an Escherichia coli-elicited coccidial protein. The coccidia are currently controlled in the poultry industry by use of anticoccidial compounds, but it now may be possible to use the bird's own immune system for defense against the parasitic infection. Since the sporozoite stage, which initiates the infection in poultry, is quite complex and is made up of hundreds of proteins or antigens, hybridoma antibodies were produced to identify specific antigens. These antigens, once identified, were found in such minute amounts that it became necessary to utilize genetic engineering in order to produce enough protein for immunization studies. One such protein, designated 5401, has been shown to stimulate an antibody response in immunized birds and to impart partial protection against a coccidial challenge infection. The results of these studies indicate that development of a vaccine against coccidial parasites may someday be possible.