We examined a panel of 25 small cell lung cancer (SCLC) cell lines and nude mouse xenografts for expression of the proto-oncogenes c-met and c-kit, and for expression of the corresponding ligands, hepatocyte growth factor (HGF) (also known as scatter factor (SF)), and stem cell factor (SCF), respectively. Expression of mRNA was detected by Northern blotting, and c-met and c-kit protein expression was detected by Western blotting and immunocytochemistry. c-met and c-kit mRNA was expressed in 22 of the examined cell lines or xenografts, and coexpression of the two proto-oncogenes was observed in 20 tumours. Expression of c-met and c-kit protein paralleled in the mRNA expression. HGF/SF mRNA was expressed in two of the examined tumours, and only one of these also expressed the c-met proto-oncogene. SCF mRNA was expressed in 19 of the examined tumours, and in 18 of these coexpression of c-kit and SCF was present. The high percentage of SCLC tumours expressing c-met and c-kit indicates that these proto-oncogenes may have an important function in this disease. The rare coexpression of c-met and HGF/SF is evidence that an autocrine regulatory pathway is not present for this receptor/ligand system in SCLC, while the frequent coexpression of c-kit and SCF indicates that this receptor/ligand system may have an autocrine function in SCLC.