This paper focuses on the influence of chemoresistance on the herpes simplex virus (HSV-tk)/ganciclovir (GCV)-induced bystander effect (BE), as studied in a human small cell lung cancer (SCLC) cell line (GLC4) and its sublines with in vitro acquired resistance to adriamycin (GLC4/ADR), mitoxantrone (GLC4/MITO) and cisplatin (GLC4/CDDP).