By semiquantitative RT-PCR, freshly isolated AA4^-/BGD6⁺ mast cells contain message for the α and β subunits of FcϵRI, mast cell–specific proteases, and carboxypeptidase A. The cDNA prepared from the same number of either total bone marrow cells (BM) or cells bound to mAb-AA4 beads (AA4) or cells bound to mAb-BGD6 beads after clearing with mAb-AA4 beads (BGD6) was serially diluted as indicated and amplified. The products were analyzed on 2.5% agarose gels. The RT-PCR confirms that the cells isolated from the bone marrow with mAb-AA4 beads are mast cells and that the undifferentiated mast cells (BGD6) contain message for FcϵRI, mast cell–specific proteases, and c-kit. FcϵRIα indicates α subunit of the high-affinity IgE receptor; FcϵRIβ, β subunit of the high-affinity IgE receptor; mouse-MCP-5, mouse mast cell protease-5; mouse-MCP-7, mouse mast cell protease-7; mouse-CPA, mouse carboxypeptidase A.