We investigated stem cell factor (SCF) expression in osteoblasts because mast cells, which occur ectopically in hyperparathyroid bone, are induced by SCF. Nontransformed osteoblasts and Saos2 or MG63 cells expressed SCF in response to PTH. Western analysis showed only large, cell-associated isoforms, M(r)s ~ 40-48 kD. Transfection of MG63 cells with plasmids expressing antisense SCF mRNA eliminated immunoreactive SCF. Sequencing osteoblast SCF cDNAs showed that exon 6 was omitted. mRNAs without exon 6 produce membrane-associated SCF isoforms in rodents, suggesting that human SCFs are processed similarly. The major osteoblastic SCF mRNA, ~ 5 kB, was augmented by PTH. Neither protein or mRNA was increased by vitamin D, however, 6-7 kB transcripts were predominant in other tissues but not detectable in osteoblasts. We conclude that osteoblasts express SCF in response to PTH, with mRNA and protein processing differences relative to other cells. SCF stimulates osteoclasts, suggesting that PTH-induced osteoblastic SCF functions to accelerate bone turnover. Mast cells may occur due to SCF overexpression at extreme PTH levels.