At least 70% of small cell lung cancer (SCLC) tumors and tumor-derived cell lines coexpress the genes for stem cell factor (SCF) and its receptor, the c-kit proto-oncogene. To assess the impact of coexpression of the growth factor and receptor on SCLC growth, the NCI-H146 SCLC cell line, which expresses only SCF, was transfected with a c-kit expression vector. Kit protein immunoprecipitated from the transfected cells had a constitutive level of tyrosine phosphorylation, and these cells grew more vigorously in serum-free medium compared to control-transfected cells. This growth advantage could be blocked by the addition of the tyrosine kinase inhibitor herbimycin A. Growth of the c-kit-transfected cells could be further enhanced by the addition of bombesin or insulin-like growth factor-1, suggesting that the SCF/c-kit autocrine loop could function cooperatively with other SCLC autocrine loops. To further investigate the importance of this autocrine loop, a cell line that naturally coexpresses SCF and c-kit was transfected with a kinase-defective c-kit gene. Cells transfected with the defective gene showed a marked decrease in their ability to grow under growth factor-free conditions compared to cells transfected with the empty expression vector. Taken together, these studies demonstrate that the coexpression of the stem cell factor and c-kit genes is a major contributor to the growth factor independence of SCLC.