Following retroviral infection, specific antibodies against viral proteins may be detected in the blood, urine, milk, or saliva of an infected animal. Enzyme-linked immunosorbent assay (ELISA) is the most commonly used method to screen for antiviral antibodies, as the technique is sensitive, reproducible, relatively inexpensive, and easily adapted to large-scale screening. This chapter describes two independent ELISA techniques. The first uses a recombinant biotinylated viral protein antigen expressed in E. coli that is purified directly on streptavidin-coated plastic wells. The second uses fixed "virus-infected" cultured cells and can be used either as a second confirmatory assay or initially in a small-scale screening to identify potentially infected individuals for further study.