Recent reports have shown that various marrow-derived cell populations respond vigorously to recombinant rat stem cell factor (rrSCF""), one form of the kitligand. In the present study, we isolated cell populations from rat bone marrow using the Thy 1.1 antigen (an antigen that in the rat is differentially expressed on primitive hemopoietic progenitor cells) and fluorescently conjugated rrSCF'64 (rrSCF'64-PE). We show that rrSCFlh4 only stimulates cells that are enriched in the brightest Thy 1.1 populations (Thy 1 .lhrlght). Numerous cell lines were generated by serial passage in rrSCF'"" containing medium, and the prototypic cell lines have been designated SKT002 and SRT003. Each cell line retains the Thy 1.1 bright phenotype and does not respond to interleukins (IL) 1-8, IL-10, granulocyte (G) colony-stimulating factor (CSF), granulocyte macrophage (GM) CSF, M-CSF, or crude preparations of mitogen-stimulated T-cell supernatants. The Thy 1 . 1 'wht population OC rat marrow was subdivided into a subset that binds rrSCF'""-PE (Thy 1 . 1 hriglht , rrSCF'"4+). The majority of these cells possess certain characteristics in common with marrow-derived mast cells and the Thy 1.1 br'ght, rrSCF'"" responsive cell lines, having similar granule morphology, being metachromatic, and reacting positively with alcian blue. Moreover, rats treated with rrSCF'64 displayed significant increases in Thy 1.1 bright, cells in the bone marrow. These studies show that the combination of Thy 1 .I and rrSCF'"" makes possible the isolation of a unique subset of rat bone marrow cells that differentially express the Thy 1.1 antigen and the cell surface receptor c-kit, the majority of which are morphologically similar to marrow-derived mast cells.