Mast cell progenitors (MCp) leave the bone marrow and migrate to peripheral tissues where they mature. Although the existence of committed MCp in adult mouse and human blood has been postulated, they have never been found. We have isolated a rare population of cells in adult mouse blood, committed to the mast cell lineage. These were identified as lineage− c-kithi ST2+ integrin β7hi CD16/32hi cells. Moreover, a major difference in maturity of these cells based on FcεRI expression was observed between the Th2-prone BALB/c strain and the Th1-prone C57BL/6 strain (66% vs 25% FcεRI+, respectively). Therefore, the choice of mouse strain is critical when studying disease models such as experimental asthma where mast cells and their progenitors are involved.