Stem cell factor (SCF), characterized as mast cell growth factor, is known to be produced by fibroblasts, keratinocytes and endothelial cells. Two different splice variants encode for either a soluble (SCF-1) or a membrane-bound (SCF-2) form. In order to explore whether mast cells themselves can produce SCF, we examined cultured cord blood (CBMC) and peripheral-blood-derived mast cells (PBMC), mast/basophil cell lines (HMC-1 and KU-812), and skin mast cells for SCF expression. On immunocytochemistry, cytoplasmatic SCF-reactivity was observed in HMC-1 cells, with additional cell membrane staining in KU-812, skin and cultured mast cells. Low amounts of SCF could be detected by ELISA in lysates of isolated and unstimulated mast cells and in supernatants of skin cells stimulated with anti-IgE or Ca-ionophore A23187. SCF mRNA was detected in all cells, although marked quantitative differences were observed among the various cell types. SCF-2 mRNA expression was low in HMC-1 cells while it was marked in skin mast cells, KU-812 cells, CBMC and PBMC. A time-dependent, increasing induction of both SCF forms was seen in CBMC and PBMC during culture. After stimulation with A23187 and phorbol myristate acetate, an up-regulation of SCF mRNA was noted in HMC-1 and KU-812 cells, without changes in the relationship of the two splice variants. The differential expression of SCF-specific mRNA splice variants in immature and mature human mast cells and the secretion of this molecule by these cells may play a role in autocrine stimulation, maintenance of survival and the differentiation of tissue mast cells.