This study aimed to identify novel markers to distinguish between murine M1 and M2 macrophages. The authors performed a comprehensive analysis of the transcriptional signatures of M0, M1, and M2 macrophages and identified genes that are common or exclusive to each subset. Real-time PCR validated the expression of CD38, G-protein coupled receptor 18 (Gpr18), and Formyl peptide receptor 2 (Fpr2) as M1-specific markers, while Early growth response protein 2 (Egr2) and c-Myc were M2-specific. Flow cytometry confirmed that M1 and M2 macrophages can be distinguished by their relative expression of CD38 and Egr2. Egr2 labeled more M2 macrophages (~70%) than the canonical M2 marker Arginase-1, which labels 24% of M2 macrophages. Conversely, CD38 labeled most (71%) in vitro M1 macrophages. In vivo, a similar CD38+ population increased after LPS exposure. Overall, this work defines exclusive and common M1 and M2 signatures and provides novel tools to distinguish M1 and M2 murine macrophages.This study aimed to identify novel markers to distinguish between murine M1 and M2 macrophages. The authors performed a comprehensive analysis of the transcriptional signatures of M0, M1, and M2 macrophages and identified genes that are common or exclusive to each subset. Real-time PCR validated the expression of CD38, G-protein coupled receptor 18 (Gpr18), and Formyl peptide receptor 2 (Fpr2) as M1-specific markers, while Early growth response protein 2 (Egr2) and c-Myc were M2-specific. Flow cytometry confirmed that M1 and M2 macrophages can be distinguished by their relative expression of CD38 and Egr2. Egr2 labeled more M2 macrophages (~70%) than the canonical M2 marker Arginase-1, which labels 24% of M2 macrophages. Conversely, CD38 labeled most (71%) in vitro M1 macrophages. In vivo, a similar CD38+ population increased after LPS exposure. Overall, this work defines exclusive and common M1 and M2 signatures and provides novel tools to distinguish M1 and M2 murine macrophages.