We have previously reported on the successful engraftment and long-term expression of human HSC (haemopoietic stem cells) in sheep following transplantation into pre-immune fetuses. The mechanism(s) underlying the support/maintenance of human haemopoiesis in sheep is not understood. In these studies we examined the ability of sheep stroma to support human haemopoiesis in vitro and compared this with support provided by human stroma. We found that, overall, human stroma was significantly more effective than sheep stroma in supporting human haemopoiesis in vitro. Nevertheless, stroma established from bone marrow of normal sheep was capable of lower-level support of the proliferation and multilineage differentiation of highly purified (CD34+, CD38-) preparations of human HSC in vitro. At lower concentrations, sheep stroma conditioned media (CM) induced human bone marrow mononuclear cells enriched for the CD34-positive fraction to enter the cell cycle at levels comparable to that of human stroma CM. However, unlike human CM, at higher concentrations, sheep CM inhibited human cells from initiating the cell cycle. These results provide a possible explanation for the long-term maintenance of low levels of human haemopoietic cell activity in human/sheep chimaeras.