Expression profiles of mRNAs and proteins for various transcription factors were determined for human skin mast cells (sMCs), leukemic HMC-1 MCs, monocytes and granulocytes. By quantitative RT-PCR, sMCs expressed lower levels of c-fos, PU.1, C/EBPalpha, and C/EBPepsilon than monocytes and granulocytes, but higher levels of MITF, SCL, GATA-1 and GATA-2. At the protein level, MITF, SCL, GATA-2, Elf-1 and c-fos were clearly detectable in sMCs. With the exception of c-fos, these proteins were absent or expressed only slightly in monocytes and granulocytes. The expression of NF-E2p45, GATA-1, PU.1, Ets-1, C/EBPalpha and C/EBPepsilon was below the detection limit in sMCs, but detectable in other myelocytes. The high expression of SCL and GATA-2 in sMCs is reminiscent of stem cells. The absence of C/EBPvarepsilon in sMCs, but strong expression in HMC-1, suggests it may impair MC maturation. In summary, mature human MCs can be characterized as C/EBPalpha(low), C/EBPvarepsilon-, PU.1(low), GATA-1(low), GATA-2+, SCL+, MITF(high).