A modified ELISA, in which a biotinylated second antibody and a streptavidin-biotinylated peroxidase complex are used, has been developed to identify the blood meal sources of black flies. The modified assay is sensitive enough to identify correctly 100% of blood meals at 73 h postingestion and 80% of blood meals at 116 h postingestion in black flies held in the field at ambient temperatures.