The proto-oncogene c-kit, encoding a receptor-type tyrosine kinase, is allelic with the W locus of the mouse. The stromal cell line OP9, capable of supporting long-term hematopoiesis, was newly established from a newborn B6C3F1-op/op mouse calvaria. When bone marrow cells of WBB6F1-W/Wv mice were cocultured with the OP9 cells in liquid medium, hematopoiesis declined to a level one-thousandth of that in the cocultures of bone marrow cells of WBB6F1-+/+ mice and stromal cells by day 21. In contrast, when bone marrow cells of W/Wv mice were cocultured with OP9 cells in semisolid medium, at least 61% of the number of colonies were detected until the end of our observation period of 42 days when compared with that in control cocultures, although colonies formed by hematopoietic stem cells of W/Wv mice were significantly smaller than those of normal stem cells. After a 24-hour incubation with OP9 cells, fewer stem cells of W/Wv mice than normal ones adhered to the stromal cells. Adhesion of normal stem cells to stromal cells was inhibited by the addition of an antagonists anti-c-kit monoclonal antibody, ACK2. These results demonstrate that the c-kit receptor plays an important role not only in the proliferative response of hematopoietic stem cells but also in their adhesion to stromal cells.