Lipopolysaccharide-induced necrosis of grafted tumors was potentiated by several hydrophilic and lipophilic muramyl dipeptide (MDP) derivatives administered a few hours prior to small amounts of lipopolysaccharide (LPS) in spite of low titers of induced circulating tumor necrosis factor (TNF). However, pretreatment with MDP derivatives did increase the level of TNF in the blood of mice challenged by a greater dose of LPS. The TNF amount in 2 h postendotoxin mouse serum reached a peak when the glycopeptide had been given 6 h before the challenge, being approximately 100-fold above that obtained in unprimed mice. The cytotoxic activity in mouse serum was inhibited by rabbit antibodies raised against recombinant mouse TNF. Although there exists a toxic synergism between BCG or MDP and endotoxin, the effect of certain MDP derivatives was not related to an increased susceptibility to the toxicity of LPS.