As an initial step to understand rapid growth of small cell lung cancer (SCLC), a complementary DNA library prepared from a SCLC cell line was screened with viral oncogene probes encoding protein-tyrosine kinases, which are known to play an important role in regulation of cell growth. Fifteen clones hybridizing with v-fms probe were isolated, and, by partial sequence analysis, four of them were identified to be c-kit protooncogenes. Northern blot study demonstrated that most of the SCLC tumors and cell lines expressed c-kit transcripts, while non-SCLC tumors and cell lines did not. Neither amplification nor rearrangement of the c-kit gene was demonstrated in SCLC cell lines by Southern blot analysis, however. Our results suggested that c-kit expression in SCLC reflects the unique biological nature of the tumor cells different from non-SCLC and further suggested that the c-kit product may participate in autocrine or paracrine stimulation of SCLC growth.