text: Promotion of tumor growth in vivo by antimacrophage agents.
  Various attempts were made to assess the role of the mononuclear phagocyte system in tumor resistance of rats in vivo. The growth of sc inoculated, weakly immunogenic, carcinogen-induced, syngeneic tumor cells was modestly reduced by ip injection and, more impressively, by local injection of peptone-induced, activated, nonimmune macrophages. A single iv injection of silica particles or carrageenan on the day of sc tumor cell inoculation greatly enhanced tumor growth. When these agents had been given a few days before or after tumor cell inoculation, the tumor-promoting efficiency was distintly diminished or even cancelled. The enhancing effects of silica and carrageenan on tumor growth were nulified by the macrophage-stabilizing agent, poly-2-vinylpyridine N-oxide, To assess the in vivo consequences of silica administration, various cellular, biochemical, and functional macrophage parameters were determined at different intervals. Results indicated the complexity of events elicited after the mononuclear phagoycte system was damaged, which made the interpretation of such results difficult.