The gene for the human interleukin-2 receptor alpha chain (IL-2R alpha) is expressed only in stimulated, not in resting, human T lymphocytes. This gene, in conjunction with others, plays a pivotal role in eliciting the T cell-mediated immune response. We have investigated the promoter and exon 1 region, the nucleotide -300 to +300 region of this gene relative to the position of transcriptional initiation at nucleotide +1, particularly with respect to the extent of DNA methylation at the 5'-CG-3' sequences and its changes upon induction. By using RNA transfer analyses and the in vivo footprinting technique, we have confirmed the previously reported finding that, upon stimulation of lymphocytes by phytohemagglutinin (PHA) plus interleukin-2 (IL-2), the IL-2R alpha gene can be induced to be transcribed. The region of the IL-2R alpha gene analyzed for 5'-CG-3' methylation by the genomic sequencing method or a polymerase chain reaction-based method subsequent to HpaII or HhaI cleavage of the DNA does not seem to be significantly methylated in most cell types tested, except for the cytidine residue in position +198 which is partly methylated. In the DNA of cells from a chronic B cell lymphatic leukemia 5'-CCGG-3' sequences in the exon 1 region are almost completely unmethylated. These results suggest that the promoter of a gene that is crucial in promoting the immune response and may have to be activated momentarily, will not be silenced by a long-term mechanism like DNA methylation. It is striking that the absence of DNA methylation in this promoter and exon 1 segment also extends to cell types not directly associated with the immune response and even to continuous cell lines.