The murine model of acquired immunodeficiency disease was used to evaluate both the antiretroviral and antiherpetic activities of the acyclic nucleotide analog 9-(2-phosphonylmethoxyethyl)adenine (PMEA). The antiretroviral activity of PMEA was compared with that of azidothymidine (AZT) in mice receiving the drug either immediately after infection or at late times in disease progression. Both AZT (oral, 30 mg/kg) and PMEA (parenteral, 25 and 5 mg/kg) were effective in slowing the development of disease when administered daily beginning on the day of infection. In contrast, neither drug alone was effective in modifying disease outcome when administered several weeks after viral infection. Human recombinant alpha interferon (rhuIFN alpha-B/D at 5 x 10(7) U/kg) was also ineffective when administered late in the course of disease. However, when administered in combination, both alpha interferon and PMEA (25 mg/kg) were able to suppress disease progression even when treatment was initiated as late as 3 weeks postinfection. Mice that were immunocompromised due to LP-BM5 virus infection were highly susceptible to acute (lethal) infection with herpes simplex virus type 1, whereas their immunocompetent littermates were not. PMEA was as effective as acyclovir in the treatment of opportunistic herpes simplex virus type 1 infections in LP-BM5 virus-infected mice. Thus, like AZT, PMEA was effective against retrovirus infection, and, like acyclovir, PMEA was effective against herpes simplex virus type 1 infection. This gives PMEA the unique potential of being useful in the treatment of opportunistic herpes simplex virus infections as well as the underlying retroviral disease.