The clear morphological differences between the granulated metrial gland (GMG) cells which develop in the uteri of rats and mice have been used to assess the origin of GMG cells in radiation chimaeras. Lethally irradiated mice were reconstituted with either rat or mouse bone marrow cells. Control mice were ovariectomised. A schedule of steroid injections was carried out allowing deciduoma formation to be induced in the irradiated and control animals. Deciduoma formation was impaired, and there were few GMG cells in the irradiated, bone marrow-reconstituted mice in comparison with the ovariectomised controls. However, it was possible to identify, by their distinctive morphology, 1) rat GMG cells in the mice reconstituted with rat bone marrow cells, and 2) mouse GMG cells in the mice reconstituted with mouse bone marrow cells. Conversely, no typical mouse or rat GMG cells were identified in mice reconstituted, respectively, with rat or mouse bone marrow cells. The observations indicate that GMG cells differentiate from a precursor derived from bone marrow.