To test the hypothesis that the omental lymphoid organ (OLO) made by peritoneal milky spots is either a source of haemopoietic (macrophage) progenitors or growth factors we attempted to culture OLO cells in vitro in a variety of assay combinations. With the culture in vitro in semisolid agar it was found that OLO cells do not form granulocyte-macrophage or macrophage colonies in response to stimulants. However, when in the same assay marrow cells were used as the targets and OLO-related preparations as stimulants it was observed that marrow cells formed exclusively macrophage colonies. These marrow cells, in response to stimulants derived from other organs, produced granulocyte-macrophage, granulocyte and macrophage colonies. OLO-related preparations tested for macrophage-colony stimulating activity included partly purified medium conditioned by OLO cells derived from mice, either injected with endotoxin or not, and medium conditioned by OLO cells after 14 days, liquid culture in vitro. While these results were observed in Swiss mice, C3H/W mice, which are genetically endotoxin-unresponsive, failed to show this reaction. These data may suggest that the local production of macrophage-colony stimulating activity in the peritoneal cavity could be one physiological role for OLO. OLO is the first organ in adult mice identified to stimulate exclusively macrophage colony growth, and not granulocyte-macrophage or pure granulocyte colonies.