The authors report preliminary results of the virological evaluation (performed by serum p24 antigen detection and HIV isolation from peripheral blood lymphocytes) of Zidovudine therapy in 64 anti-HIV positive subjects. Data presented indicate that the administration of therapeutic doses of the drug can efficiently and precociously inhibit viral replication, suggesting that the clinical benefits of the therapy may be related to the anti-viral activity of the drug. However, the anti-viral efficacy of Zidovudine seems to be of limited length; in fact a reactivation of viral replication was observed in most patients during the study.