Murine AIDS (MAIDS) is caused by a defective retrovirus present in the LP-BM5 murine leukemia virus mixture. Strains of inbred mice differ in resistance to MAIDS development; some are susceptible (e.g., C57BL/6), while others are resistant (e.g., CBA and B10.BR). As an early block to viral replication in resistant mice has been demonstrated previously by PCR studies, we postulated that alpha/beta interferons (IFN-␣/) may be involved in resistance to MAIDS. Susceptible C57BL/6 mice infected with LP-BM5 were treated with IFN-␣/ or Newcastle disease virus. Newcastle disease virus induces high endogenous IFN-␣/ production in mice. Both treatments delayed the development of MAIDS, as assessed by splenomegaly and T-and B-cell proliferation. In addition, an IFN-␣/ response was detected by reverse transcription-PCR and dot blotting 3, 6, and 9 h after LP-BM5 infection in resistant mice but not in susceptible mice. These results suggest that the ability to produce IFN-␣/ in response to LP-BM5 infection may contribute to host resistance to MAIDS.