We have demonstrated that biologically active muramyl peptides, in particular, glucosaminylmuramyl dipeptide (GMDP), augmented in vitro cytotoxic activity of tumor necrosis factor-α (TNF-α) against murine fibrosarcoma L929 cells. The introduction of GMDP resulted in cytotoxic effect characteristic for substantially higher dose of cytokine. Even more potent was the combination of GMDP, TNF-α and Actinomycin D (ActD). According to clonogenic and MTT assays 100% L929 cells could be killed in culture with low doses of TNF-α and ActD if GMDP was present. When cisplatin was substituted for ActD similar results were obtained. GMDP also enhanced cytotoxicity of TNF-α and cisplatin against human breast carcinoma MCF7 and histiocytic lymphoma U937 cells. Normal cells, namely human peripheral blood leucocytes and murine peritoneal macrophages, were resistant to selected doses of TNF-α/cisplatin/GMDP. © 2005 Elsevier B.V. All rights reserved.