Several studies have reported that bone marrow (BM) cells may give rise to neurons and astrocytes in vitro and in vivo. To further test this hypothesis, we analyzed for incorporation of neural cell types expressing donor markers in normal or injured brains of irradiated mice reconstituted with whole BM or single, purified c-kit⁺Thy1.1^loLin⁻Sca-1⁺ (KTLS) hematopoietic stem cells (HSCs), and of unirradiated parabionts with surgically anastomosed vasculature. Each model showed low-level parenchymal engraftment of donor-marker⁺ cells with 96–100% immunoreactivity for panhematopoietic (CD45) or microglial (Iba1 or Mac1) lineage markers in all cases studied. Other than one arborizing structure in the olfactory bulb of one BM-transplanted animal, possibly representing a neuronal or glial cell process, we found no donor-marker–expressing astrocytes or non-Purkinje neurons among >10,000 donor-marker⁺ cells from 21 animals. These data strongly suggest that HSCs and their progeny maintain lineage fidelity in the brain and do not adopt neural cell fates with any measurable frequency.