In the past, positive as well as negative results pertaining to HIV-1/HIV-2 interference have been obtained. Therefore, in the present study attention was paid to the viral expression state of preinfected cells at the time of exposure to secondary virus. A clonal HIV-2 infected HUT-78 cell line was derived by endpoint dilution and subsequently inoculated with cell-free HIV-1. Superinfection with HIV-1 was ruled out by Western blot and PCR analysis. The chronically HIV-2 infected cells used for these studies showed a highly productive expression state, as evidenced by immunoperoxidase staining (IPS), Western blot profile and levels of reverse transcriptase (RT) activity. We discuss several mechanisms of interference in productively infected cells, which may confer resistance to superinfection with secondary virus.