Thioglycollate (TG)-elicited peritoneal macrophages (mϕs) were highly proliferative and formed mϕ colonies in vitro in the presence of mϕ colony-stimulating factor (M-CSF), while resident peritoneal mϕ did not. To determine whether such proliferative mϕs are immigrant or locally activated resident mϕs, mice depleted of bone marrow cells and circulating monocytes by bone- seeking radiostrontium (89Sr) were injected intraperi- toneally with TG. For control (88Sr) and splenectomized (Spx) mice, more than 4 × 10−4 mϕ colony-forming cells (M-CFCs) per mouse were recovered in the peritoneal lavage fluid 5 days after TG injection. 89Sr-treated mice, on the other hand, had only 20% of those in the control mice. Splenectomized and 89Sr-treated (Spx/89Sr) mice showed further depletion of bone marrow cells and monocytes and, as expected, total numbers of peritoneal M- GFCs were severely depressed to less than 1% of those in the control mice. The results suggest that levels of peritoneal M-GFCs are strongly dependent on the presence of radiosensitive bone marrow cells and circulating monocytes, and resident peritoneal mϕs activated locally by inflammatory stimuli do not form mϕ colonies under the defined conditions.