Interferon gamma (IFN-gamma) has pleiotropic biological effects, including intrinsic antiviral activity as well as stimulation and regulation of immune responses. An infectious recombinant human respiratory syncytial virus (rRSV/mIFN-gamma) was constructed that encodes murine (m) IFN-gamma as a separate gene inserted into the G-F intergenic region. Cultured cells infected with rRSV/mIFN-gamma secreted 22 microg mIFN-gamma per 10(6) cells. The replication of rRSV/mIFN-gamma, but not that of a control chimeric rRSV containing the chloramphenicol acetyl transferase (CAT) gene as an additional gene, was 63- and 20-fold lower than that of wild-type (wt) RSV in the upper and lower respiratory tract, respectively, of mice. Thus, the attenuation of rRSV/mIFN-gamma in vivo could be attributed to the activity of mIFN-gamma and not to the presence of the additional gene per se. The mice were completely resistant to subsequent challenge with wt RSV. Despite its growth restriction, infection of mice with rRSV/mIFN-gamma induced a level of RSV-specific antibodies that, on day 56, was comparable to or greater than that induced by infection with wt RSV. Mice infected with rRSV/mIFN-gamma developed a high level of IFN-gamma mRNA and an increased amount of interleukin 12 p40 mRNA in their lungs, whereas other cytokine mRNAs tested were unchanged compared with those induced by wt RSV. Because attenuation of RSV typically is accompanied by a reduction in immunogenicity, expression of IFN-gamma by an rRSV represents a method of attenuation in which immunogenicity can be maintained rather than be reduced.