In an attempt to predict the antitumor activity of a new podophyllotoxin analogue, NK 611, in the treatment of lung cancer, we compared the drug with etoposide and teniposide using four human small cell lung cancer (SCLC) cell lines, SBC-2, -3, -4, -7, and two non-small cell lung cancer cell lines, ABC-1, EBC-1. In terms of the fifty percent tumor growth inhibitory concentration (IC 50) determined by MTT assay, teniposide was most potent among the drugs. The degree of cross-resistance of each drug was investigated using an etoposide-resistant SCLC subline (SBC-3/ETP), an adriamycin-resistant subline (SBC-3/ADM 100), and a cisplatin-resistant subline (SBC-3/CDDP). As for relative resistant (the ratio of IC 50 for resistant subline to that for the parent subline), NK 611 was least cross-resistant to etoposide, adriamycin, and cisplatin among drugs tested. These results indicate that NK 611 may play a role in a salvage chemotherapy for patients with resistant SCLC.