Factors influencing granulopoiesis have been evaluated using diffusion chambers implanted in the peritoneal cavity of rabbits. An increase in granulopoiesis in chambers implanted in hosts made neutropenic by nitrogen mustard occurs in mice made neutropenic by x-ray or drug. The intraperitoneal injection of leukocytes inhibited the growth of cells in chambers implanted in rabbits. Removal of mature granulocytes from marrow prior to chamber inoculation produced a marked in-increase in cell growth, especially of granulocytes. Mature granulocytes clearly inhibited cell replication and this inhibition involved both myeloid and erythroid elements, although the data suggest a greater effect on myelopoiesis. In contrast to the mouse, erythropoiesis in chambers in rabbits remained prominent for over 1 wk.