AbstractRen-shen-yang-rong-tang (Japanese name: Ninjin-youei-to, NIN), a traditional Chinese medicine, is a drug made of spray-dried powder of hot water extract obtained from twelve species of medical plants. An intraperitoneal (ip) injection with NIN 2 days before intravenous (iv) infection withListeria monocytogenes (L. monocytogenes)accelerated elimination of viable bacteria in the spleen in the early stage of infection (from day 1) and protected mice from the lethal infection. It was suggested that the protective effect of NIN was mediated by the activation of nonimmune macrophages playing a principle role in resistance in the early stage of infection. Two days after ip injection with NIN just before infection, significantly increment in the number of monocytes in the peripheral blood was observed, though macrophage number in the spleen and their intracellular killing activity were unchanged. At 12 hours after infection withL. monocytogenesa significantly enhanced increase of splenic macrophage number was observed in NIN-treated mice, compared to controls. After ip injection of NIN, interleukin-1 (IL-1), IL-6 and granulocyte macrophage-colony stimulating factor (GM-CSF) became detectable in the serum or peritoneal cavity. These results suggested that NIN stimulated macrophage-precursor cells in the bone marrow via the production of IL-1, IL-6, GM-CSF by macrophages, accelerated the supply of peripheral macrophages, and such macrophages accumulated into the site of infection in the very early stage of infection. Similar protective effects of NIN were observed by oral administration for 7 days till 1 day before iv infection withL. monocytogenes.