Drug resistance is one of the major impediments to the treatment of advanced neuroblastoma. Two neuroblastoma cell lines established from the same patient before (KP-N-AY) and after (KP-N-AYR) chemotherapy are described. Both cell lines were established from bone-marrow metastases of a 2 1/2-year-old patient with stage IV neuroblastoma. Chromosomal analysis, catecholamine assessment and the surface membrane phenotype of these cell lines confirmed that the tumors were of neuroblastoma origin. Compared with the KP-N-AY cell line, the KP-N-AYR line had decreased N-myc amplification but incresed N-myc expression. An in vitro sensitivity test using a clonogenic assay showed the KP-N-AYR cell line to be 3.0-fold resistant to adriamycin and 2.7-fold resistant to cis-platinum as compared with the KP-N-AY cell line. The expression of the multi-drug-resistance gene (MDRI) was not observed in either cell line by the ribonuclease protection assay. The KP-N-AY cell line revealed only faint MDRI RNA by the polymerase chain reaction, whereas the KP-N-AYR cell line had no expression of the MDRI gene. The level of glutathione-S-transferase-pi-was significantly higher in the KP-N-AYR cell line than in the development of clinical drug resistance may be associated with the enhanced glutathione-S-transferase-pi activity but not with MDRI gene expression.