The DNA in mammalian genomes is characterized by complex patterns of DNA methylation that reflect the states of all genetic activities of that genome. The modified nucleotide 5-methyldeoxycytidine (5mdC) can affect the interactions of specific proteins with DNA sequence motifs. The most extensively studied effect of sequence-specific methylations is that of the long-term silencing of eukaryotic (mammalian) promoters. We have initiated studies on the methylation status of parts of the human genome to view patterns of DNA methylation as indicators for genetic activities. In this report, analyses using both restriction enzyme--Southern blotting and the very precise genomic sequencing technique have been done. The genes for tumor necrosis factors (TNF) alpha and beta--in particular, their 5'-upstream and promoter regions--have been investigated in DNA isolated from human lymphocytes, granulocytes, and sperm. The results are characterized by a remarkable interindividual concordance of DNA methylation in specific human cell types. The patterns are identical in the DNA from one cell type for different individuals even of different genetic origins but different in the DNA from different cell types. As an example, in the DNA from human granulocytes of 15 different individuals (ages 20-48 yr, both sexes), 5mdC residues have been localized by the genomic sequencing technique in three identical sequence positions in the 5'-upstream region and in one downstream position of the gene encoding TNF-alpha. The promoter of this gene is free of 5mdC, and TNF-alpha is expressed in human granulocytes. The TNF-beta promoter is methylated in granulocytes from 9 different individuals, and TNF-beta is not expressed. In human lymphocytes, the main source of TNF-beta, the TNF-beta promoter is free of 5mdC residues. All 5'-CG-3' sites studied in the TNF-alpha and -beta genes are methylated in DNA from human sperm. In human cell lines HL-60, Jurkat, and RPMI 1788, the extent of DNA methylation in TNF-alpha and -beta genes has also been studied.