T o determine the effect of aging on IFN-γ–induced MHC class II antigen expression, we produced bone marrow–derived macrophages in vitro. In these conditions, we analyzed the effect of aging on the genomic expression of macrophages without the influence of other cell types that may be affected by aging. Although macrophages from young and aged mice showed an identical degree of differentiation, after incubation with IFN-γ, the expression at the cell surface of the IA complex and the levels of IAβ protein and mRNA were lower in aged macrophages. Moreover, the transcription of the IAβ gene was impaired in aged macrophages. The amount of transcription factors that bound to the W and X, but not to the Y, boxes of the IAβ promoter gene was lower in aged macrophages. Similar levels of CIITA mRNA were found after IFN-γ treatment of both young and aged macrophages. This shows that neither the initial cascade that starts after the interaction of IFN-γ with the receptor nor the second signals involved in the expression of CIITA are impaired in aged macrophages. These data indicate that aging is associated with low levels of MHC class II gene induction by IFN-γ because of impaired transcription.