Proteinase-3/Myeloblastin (Mbn) is a neutral serine protease and a major constituent of the primary granules of myeloid cells. It can degrade extracellular matrix proteins and has been discussed as a key factor for the initiation of terminal differentiation in promyelocytic cells. Regulation of Mbn closely parallels that of another major primary granule protein, myeloperoxidase (MPO). We examined the expression and DNA methylation of Mbn in a model of in vitro differentiation of CD34+ enriched peripheral blood progenitor cells (PBPCs), and in various other myeloid and non-myeloid tissues. Mbn mRNA was undetectable in uncultured PBPCs but was upregulated during their in vitro differentiation. Its expression was enhanced in the presence of G-CSF. Mbn expression was also detected in several myeloid cell lines but not in mature granulocytes, monocytes and macrophages. Partial demethylation at a CpG site within Mbn intron 1 (analyzed by restriction with SmaI) was observed during continued in vitro differentiation of PBPCs. This site was fully demethylated in mature granulocytes, monocytes and macrophages. Variable methylation of this site and a second SmaI site located upstream of the putative Mbn promoter region was present in other myeloid and non-myeloid tissues examined.