There is growing interest in the use of bone marrow cells to treat liver fibrosis, however little is known about their anti-fibrotic efficacy or the identity of their effector cell(s). Sphingosine-1-phosphate (S1P) mediates egress of immune cells from the lymphoid organs into the lymphatic vessels; we investigated its role in the response of hematopoietic stem cells (HSC) to liver fibrosis in mice.