1. Various chemical agents (saline, protein, or sugar solutions) injected intraperitoneally into normal and ascites tumor-bearing mice induced in the peritoneal exudate of both groups a cellular response which had essentially the same cyclic pattern—an increase of granulocytes with a disappearance of mast cells, and a subsequent return to the normal level. These agents had no effect on the tumor cell concentration in most animals. The patterns of the peritoneal cellular exudate were stable and indicated homeostatic regulation.

  2. Repeated intraperitoneal inoculations with 107 tumor cells did not raise the tumor cell concentration significantly above the level induced by a single inoculation, but considerably increased the amount of implantation into the peritoneal wall; conversely repeated withdrawals of 105 cells reduced the extent of implantation but not the free tumor-cell growth. Similar results were obtained when the amount of implantation was increased experimentally (by multiple punctures of the peritoneal wall).

  3. It is concluded that in ascites tumor-bearing mice a homeostatic mechanism operates in the peritoneal cavity to regulate the stability of the tumor cell concentration in the ascitic fluid; excessive numbers of free cells are eliminated from the fluid by implantation into the peritoneal tissue; vice versa, the loss of free cells from the fluid is compensated by a decrease in the number of implanted cells.

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This work was supported by Grant-in-aid No. CH 25 B from the American Cancer Society, New York, N.Y., and by Grant No. C2080 from the National Cancer Institute, Bethesda, Maryland.

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