The DNA methylation status of the 5'-regulatory region of human tumor necrosis factor-alpha (TNF-alpha) gene was examined using two human monocytic cell lines representing different differentiation stages: one cell line, THP-1, represents the most differentiated macrophage-like phenotypes and the other, HL-60, represents a least differentiated phenotype. Two restriction enzymes were used to detect methylated sites in the 5'-regulatory region of the TNF-alpha gene. The restriction enzyme Msp I, which recognizes both CCGG (unmethylated) and C(me)CGG (methylated) sequences, whereas Hpa II recognizes only the unmethylated CCGG sequence. Two Msp I sites located at -600 and +200 nucleotides, respectively, from the transcriptional initiation site were unmethylated in DNA from the THP-1, whereas these sites were methylated in the one from the HL-60. Treating the HL-60 cells with 1,25-vitamin D3, ( 1,25-(OH)2D3), which is known to induce monocytic cell differentiation, induce demethylation of the sites, suggesting the methylation status was correlated with the differentiation stages of monocytes.