Mifamurtide (liposomal muramyl tripeptide phosphatidylethanolamine; trade name Mepact) is an immunomodulator with antitumor effects that appear to be mediated via activation of monocytes and macrophages. After intravenous administration, mifamurtide is selectively phagocytosed by monocytes and macrophages. Cytosolic Mifamurtide specifically interacts with nucleotide-binding oligomerization domain 2 (NOD2) receptor that induces nuclear factor (NF)-kB activation and is implicated in innate immune defense. Activation of monocyte-mediated tumoricidal function was observed following in vivo treatment with mifamurtide in phase I/II clinical trials. Intravenous administration of mifamurtide inhibited tumor growth and increased survival in rodent models of lung and liver metastasis. In a large, randomized, open-label, multicenter, phase III trial, the addition of adjuvant (postoperative) mifamurtide to three- or four-drug combination chemotherapy (doxorubicin, cisplatin, and high-dose methotrexate with, or without, ifosfamide) was associated with a statistically significant improvement in overall survival in patients with newly diagnosed, high-grade, non-metastatic, resectable osteosarcoma. The pattern of outcome was generally similar in a small cohort of patients with metastatic disease who were enrolled in this trial. Mifamurtide is generally well tolerated; adverse events attributed to administration of the drug include chills, fever, headache, nausea, and myalgias. In the EU, mifamurtide is indicated in children, adolescents, and young adults for the treatment of high-grade, resectable, non-metastatic osteosarcoma after macroscopically complete surgical resection; it is administered by intravenous infusion in conjunction with postoperative multiagent chemotherapy. In the US, mifamurtide is currently an investigational agent that holds orphan drug status for the treatment of osteosarcoma.