Patients with AIDS and AIDS-related complex often show symptoms of Epstein-Barr virus (EBV) reactivation. Several EBV-encoded trans-acting factors activate the EBV lytic cycle, and one, ZEBRA (BamHI Z EBV replication activator; also called EB1), switches EBV from its latent to productive cycle. Indirect immunofluorescence studies were done using human cells transfected with a recombinant DNA-harboring cDNA sequence spanning BZLF1 (BamHI Z left frame 1) that was inserted downstream of the adenovirus major late promoter. IgG anti-ZEBRA antibodies were detected in a high proportion of asymptomatic HIV carriers and in AIDS patients but were absent in healthy control individuals. The presence of anti-ZEBRA antibodies in the sera of HIV-positive patients favors the hypothesis that EBV reactivates in such subjects. This finding may be of practical importance in the prognostication of AIDS development.