We evaluated the antiproliferative effect of L-myc antisense DNA in NCI-H209, a human small cell lung cancer (SCLC) cell line overexpressing the L-myc gene. The synthetic DNA used in the present study was oligodeoxynucleoside phosphorothioate, which showed rapid incorporation into NCI-H209 cells and localized mainly in the cell nucleus and weakly in the cytoplasm. The exposure of this cell line to L-myc antisense DNA covering the translational initiation site of L-myc proteins inhibited the cell proliferation in a dose-dependent sequence-specific manner. Furthermore, the growth inhibition by this antisense DNA was correlated with the level of L-myc expression in three SCLC cell lines, NCI-H209, NCI-H510, and NCI-H82. In Western blot analysis, expression of the L-myc proteins was down-regulated in the antisense-treated cells compared with control-treated cells in NCI-H209. Together with unique characteristics of the L-myc gene, including: (a) a frequently amplified and overexpressed state in SCLC; and (b) very restricted and low-level expression in human adult tissues, the present data indicate that L-myc is a good candidate for the target gene for antisense DNA therapy based on molecular biological diagnosis in SCLC.