In this paper we report that macrophages can be stimulated to express detectable levels of IFN-γ-specific mRNA. Macrophages from lipopolysaccharide (LPS)-responsive, C3H/OuJ mice are induced by LPS to increase steady-state levels of IFN-γ-specific mRNA, while those from LPS-hyporesponsive C3H/HeJ mice are not. This interstrain variation is apparently the result of LPS-specific signal differences since macrophages derived from both Lpsⁿ and Lps^d mouse strains are able to produce comparable levels of IFN-γ-specific mRNA following stimulation with polyinosinic-polycytidylic acid. The identity of the cell type responsible for this IFN-γ message appears to be the macrophage as IFN-γ-specific mRNA was also detectable following T and natural killer cell depletion, in the LPS-stimulated RAW 264.7 cell line, and in a homogeneous population of mature macrophages propagated in vitro by stimulation of bone marrow progenitors with recombinant colony stimulating factor-1. Immunofluorescent staining of fixed and permeabilized LPS-stimulated macrophages confirmed the presence of immunoreactive IFN-γ protein. The possible significance of IFN-γ production by macrophages is discussed in the context of normal macrophage differentiation as well as the inflammatory immune response.