Amplification and expression of 16 protooncogenes were examined in 12 established small cell lung cancer (SCLC) cell lines. Seven of 12 cell lines showed a 20- to 35-fold amplification of the c-myc oncogene, 3 cell lines showed an 80- to 130-fold amplification of N-myc oncogene, and one cell line had a simultaneous amplification of the c-myb and N-myc oncogene. In this cell line both oncogenes were transcriptionally highly active at the same time. A variant subpopulation of SCLC expressed an 8.5-kilobase v-fms homologous transcript at high levels but without amplification of the c-fms gene. All cell lines examined had similar RNA levels of the N-ras, Ki-ras, Ha-ras, and c-raf1 oncogenes. DNA amplification, however, was undetectable. The protooncogenes c-fes, c-fos, and c-erbB were expressed very weakly and the transcripts of the oncogenes c-mos, c-sis, c-erbA, c-src, and c-abl were not observed in any of the 12 SCLC-cell lines. From these data we conclude that beyond the oncogenes myc and myb, oncogenes whose gene products are GTP binding proteins and phosphokinases may also be necessary to develop and keep the malignant state of SCLC. The v-fms homologous transcript found may be involved in the transition of the classic cell type to the variant cell type of SCLC.