In order to identify novel mediators synthesized in activated macrophages, a cDNA library was prepared from cultures of the mouse macrophage cell line RAW 264.7 that had been treated with lymphokine-rich conditioned medium from mitogen-stimulated mouse spleen cells. Differential plaque hybridization identified a cDNA, designated m119, that detected a 1.6-kilobase mRNA that accumulated in response to gamma-interferon (IFN-gamma) but not in response to other macrophage activators, including IFN-alpha, IFN-beta, and lipopolysaccharide. The mRNA encoded a predicted protein of Mr 14,461 containing a 21-amino acid signal peptide. The primary structure of the predicted protein indicated that it is a member of a recently described family of cytokines related to platelet factor 4, including Gro/melanoma growth stimulatory activity and neutrophil-activating peptide/interleukin 8. The selective induction of the m119 mRNA by IFN-gamma that the predicted m119 protein mediates a macrophage activity regulated by IFN-gamma. The m119 protein may be a cytokine that affects the growth, movement, or activation state of cells that participate in immune and inflammatory responses. It is proposed that the gene encoding this protein be called mig, for monokine induced by gamma interferon.