We have previously established Adriamycin-resistant HOB1 cell lines showing the multidrug resistance (MDR) phenotype. For further study, we analyzed the free-radical scavengers glutathione S-transferase (GST) and glutathione peroxidase (GPX) by enzyme assays and Northern blots. Three cell lines, HOB1/ADR0.1, HOB1/ADR1.0, and HOB1/ADR5.0, represented HOB1 cells resistant to 0.1, 1.0, and 5.0 microM Adriamycin, respectively. The mdr1 transcript was overexpressed in HOB1/ADR0.1 cells, and the amount of its expression reached a maximum between HOB1/ADR1.0 and HOB1/ADR5.0 cell lines. The increases in GST activity and GST-pi expression were observed only in high-level-resistant cell line (HOB1/ADR1.0 and HOB1/ADR5.0), which also showed increased GPX activity and expression. For investigation of the cytotoxic effect of Adriamycin on HOB1 cells prior to the mdr1 overexpression, an appropriate number of parental HOB1 cells were treated with 0.1 microM Adriamycin for 7 days, and the viable cells (HOB1/ADR) were isolated and subjected to analyses for mdr1, GST-pi, and GPX expression and for GST and GPX activity. In comparison with HOB1/ADR0.1 cells, HOB1/ADR cells did not show mdr1 overexpression but had significant increases in the activity and expression of GST and GPX. The current study suggests that in the early phase of Adriamycin treatment, GST and GPX are more important than P-glycoprotein for the development in HOB1 cells of resistance against Adriamycin.