Comparative effect of muramyl dipeptide in vivo and in vitro on the tumoricidal activity of rat peritoneal macrophages

Muramyl dipeptide (MDP) has been tested for its ability to induce rat peritoneal macrophage tumoricidal activity. Rat resident macrophages were activated in vivo and in vitro. Nevertheless, they were cytolytic when harvested 1 or 2 h after an intraperitoneal injection of MDP, but noncytolytic when harvested later. The activation obtained in vivo and in vitro with small amounts of MDP was not increased with larger doses. As, under the same conditions, mouse peritoneal macrophages were not activated, the sensitivity of the peritoneal macrophages to MDP depends on the species used.