Abstract The effects of prolonged exposure to 2 and 10 μM 6-mercaptopurine (6MP) in the human lymphoblastic T-cell line MOLT-4 were studied with respect to cell-kinetic parameters, phosphoribosyi pyrophosphate (PRPP) and purine ribonucleotide levels, formation of 6MP-nucleotides, especially methyl-thio-IMP (Me-tIMP), DNA and RNA synthesis ([ 32P] incorporation), and [8- 14C]6MP incorporation into newly synthesized DNA and RNA. The results provided new insights into the complex mechanism of action of 6MP in human malignant lymphoblasts. Exposure to 2 μM 6MP resulted in a rapid inhibition of purine de novo synthesis (PDNS) by increased levels of Me-tIMP, resulting in increased PRPP levels and decreased purine ribonucleotides, affecting cell growth and clonal growth, and less cell death. DNA synthesis decreased, associated with an increasing delay of cells in S phase. Incorporation of thioguanine nucleotides into newly synthesized DNA resulted in an increasing arrest of cells in G 2 + M phase. RNA synthesis, initially decreased, recovered partially, associated with a recovery of purine ribonucleotides. New formation of 6MP-nucleotides (tIMP) was only detected within the first 24 hr, and 6MP levels in the culture medium were already undetectable after 6 hr of exposure to 2 μm, indicating a high rate of incorporation and complete conversion of 6MP within this period. Incorporation of 6MP-nucleotides into DNA was 5 times as high as incorporation into RNA. Exposure to 10μM 6MP resulted in early cytotoxicity at 24 hr, associated with a complete inhibition of PDNS by a large pool of Me-tIMP and lower levels of purine ribonucleotides as compared to 2 μM 6MP. A more severe delay of cells in S phase was associated with an inhibition of DNA synthesis to 14% of control within the first 24 hr, and an arrest in G 2 + M phase. Further increasing levels of Me-tIMP caused an arrest of cells and late cytotoxicity in S phase at 48 hr, preventing further progression into G 2 + M phase. Our data suggest that inhibition of PDNS due to Me-tIMP is a crucial event in the mechanism of 6MP cytotoxicity. It is responsible for decreased RNA synthesis and decreased availability of natural deoxy ribonucleotides, causing a delay of DNA synthesis in S phase. This enhances incorporation of 6MP as thioguanine nucleotides into DNA in the S phase and subsequent late cytotoxicity in the G 2 phase. However, with high concentrations of 6MP, the large pool of Me-tIMP causes severe reduction of natural deoxyribonucleotides in lymphoblasts with an active PDNS. This is responsible for pronounced inhibition of DNA synthesis and early cytotoxicity in the S phase.