Small cell lung carcinoma (SCLC) is characterized by rapid development of resistance to drugs, such as cisdiamminedichloroplatinum(II) (cDDP) and anthracyclines. The molecular basis for resistance to cDDP and adriamycin (Adr) is poorly understood. One of the genetic alterations observed in SCLC, which is correlated with poor prognosis, is amplification and overexpression of c-myc oncogene. Therefore, activation of the c-myc oncogene might form a basis for resistance. The relationship between c-myc and cDDP as well as Adr resistance was analyzed by down-regulation of endogenously expressed c-myc in the human SCLC cell line GLC4, and its cDDP and Adr resistant sublines (GLC4-cDDP and GLC4-Adr). Cells were incubated with an unmodified antisense (AS) oligodeoxynucleotide complementary to the first 5 codons of the c-myc mRNA in serum free culture medium. Pre-incubation with 15 μM AS c-myc, reduced c-myc protein expression and induced 30-35% growth inhibition in all 3 cell lines. It resulted in increased cDDP sensitivity in GLC4-cDDP but not in GLC4. However, this pre-incubation did not affect Adr sensitivity in all lines. The effect of AS c-myc pretreatment on cDDP resistance was not mediated by changes in cell cycle distribution. These findings suggest that c-myc plays a role in cDDP resistance by effects other than those on cell cycle distribution.