Antibodies to screwworm, Cochliomyia hominivorax (Coquerel), antigens are found in serum of previously infested host sheep and for at least one month after infestation. ELISA provides a serological technique for detecting previous exposure to screwworm in noninfested hosts. The technique is adaptable to a field situation and has potential as a method for screening noninfested hosts for indirectly estimating infestation rates.