To study the relationship between methylation and the transcriptional activity of the minimal promoter of the glutathione S-transferase GSTP1 gene encoding glutathione S-transferase P1-1, GSTP1 mRNA levels as well as basal promoter activity were compared in human leukemia cell lines. The K562 erythroleukemia cell line presented a strong GSTP1 promoter activity, as measured in transient transfection assays using a luciferase reporter plasmid, and correlated with a high mRNA whereas in Raji cells no mRNA was expressed. In order to establish a relationship between the expression and the methylation status, we used in vitro bisulfite sequencing which indicated that both methylated and unmethylated GSTP1 promoter alleles coexisted in K562 cells, whereas Raji lymphoma cells showed a nearly uniform hypermethylation of the promoter region. To determine the impact of methylation, we used in vitro SssI methylation of the minimal GSTP1 promoter, which led to the silencing of the promoter activity in transient transfection assays in expressing K562 as well as in non-expressing Raji cells. These data are in good agreement with previously obtained results and indicate that methylation of CpG sites of the basal promoter is an essential mechanism in the control of GSTP1 gene expression in human leukemia.