We report that stem-cell factor (SCF), the ligand of the receptor encoded by the c-kit proto-oncogene, is a potent activator of degranulation of rat peritoneal mast cells in vitro and in vivo. Freshly isolated, purified mast cells were relatively unresponsive to SCF (4-500 ng/ml) but progressively acquired responsiveness to this agent, assessed as serotonin (5-HT) release, during 48 hr culture in vitro. The cells showed a similar kinetic pattern of acquisition of responsiveness to anti-IgE but responded fully to calcium ionophore A23187 or compound 48/80 regardless of time in culture. Acquisition of mast cell responsiveness to SCF or anti-IgE was not due to serum factors or to recovery from the Percoll purification procedure. During culture, mast cell expression of the SCF receptor (SCFR) increased, and this may explain in part the increased responsiveness to SCF. However, surface IgE expression remained constant, and the increased responses to anti-IgE therefore must reflect changes in components of the secretion-coupling pathway that are activated subsequent to IgE cross-linking. The unresponsiveness of freshly isolated peritoneal mast cells to SCF or anti-IgE does not reflect a state of in vivo unresponsiveness, as peritoneal mast cells degranulated in vivo in response to these agents. We conclude that in terms of their responsiveness to SCF or anti-IgE, cultured tissue mast cells may be more representative than freshly isolated mast cells of secretory function in vivo, and therefore may be more appropriate for physiological or pharmacological studies of SCF- or IgE-dependent secretory responses.