The response of macrophages to agents such as lipopolysaccharide (LPS) and interferon (IFN) includes the transcriptional activation of numerous genes. We have used the method of differential screening of a RAW 264.7 macrophage cell line cDNA library to isolate and characterize LPS-induced messages. One such message, LRG-47, is induced by LPS, IFN-gamma, and IFN-alpha/beta, but not by a panel of other cytokines or pharmacological activating agents. LRG-47 is homologous to two other IFN-gamma-induced genes, IRG-47 and Mg21. The LRG-47 sequence is approximately 33% identical and 52% similar to both these putative protein products. All three putative proteins, particularly Mg21, bear homology to a T cell product, Tgtp, induced by T cell receptor cross-linking. The three macrophage-derived proteins share areas of homology with GTP-binding proteins, are approximately 415 amino acids in length, and have similar kinetics of induction by IFN-gamma. This suggests that these genes may be members of a new family of IFN-inducible proteins.