The report of pluripotential hemopoietic stem cells (ce:simple-paraHSC) in the murine brain, as detected in the colony-forming unit (CFUs) assay (Bartlett 1982), prompted us to investigate the functional significance of these cells. Although we confirmed that the transfer of brain cells into irradiated hosts produced CFUs, they did not reconstitute erythropoietic or immune functions. Failure to reconstitute immune function was corroborated by FACS analysis. To test whether the injection of brain cells stimulates self-reconstitution, the effect of host irradiation on CFUs production was studied. CFUs frequencies resulting from the transfer of brain or irradiated brain cells were similar and significantly higher in 800 R vs. 1200 R recipients at 10 days. Our experiments do not support the existence of PHSC in the brain, and suggest that colony formation after brain cell transfer may be the result of an adjuvant effect.