In order to evaluate the influence of antiviral nucleoside analogues upon the natural immune system, we investigated the immunomodulatory activity of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a nucleotide analogue with potent anti-HIV and anti-herpes activity, in a murine system. C57BL/6 mice were inoculated intraperitoneally with 10, 25 and 50 mg PMEA/kg. Mononuclear cells were isolated from their spleens, and some natural immune functions were evaluated. The results show that PMEA significantly increases the levels of natural killer (NK)-cell cytotoxicity. We also found that alpha/beta IFN production was substantially increased in PMEA-treated mice, while both IL-1 and IL-2 production was decreased. Thus, PMEA can increase some natural immunity functions, such as NK activity and IFN production. These results suggest that PMEA might be active in vivo against HIV and herpes viruses both as an immunomodulator and as an antiviral compound.