Most individuals can be diagnosed as being infected with HIV-1 with an enzyme immunoassay (EIA), which detects antibodies to the virus. Repeatedly positive results must be confirmed, usually by Western blot. These assays have been improved since their introduction in 1985. When used together, the EIA and Western blot are now extremely reliable, with sensitivities of 99.5% and specificities of 99.8% for the diagnosis of HIV-1 (1). These tests are fairly rapid, reproducible across laboratories, well quality-controlled, relatively inexpensive, and form the basis of the protection of the blood supply.