BTK, the gene that is defective in X-linked agammaglobulinemia, encodes a cytoplasmic tyrosine kinase that is critical for B-cell proliferation, or survival. To identify regulatory elements that control the expression of BTK we evaluated the methylation pattern of this gene in cell lines and in freshly isolated cells. An Hpa II site that was specifically demethylated in mature B cells but not in pre-B cells, T cells, neutrophils, or nonhematopoietic cells was identified in the tenth intron of BTK. In a 40 kilobase (kb) segment of DNA spanning the entire coding region of BTK plus 3 kb upstream of the first exon there were no other sites that demonstrated lineage-specific demethylation. The B-cell-specific demethylation site in intron 10, which falls within the SH2 domain, 26 kb distal to the first exon, occurs in a region rich in regulatory elements including two E2 boxes, two AP-2 sites, and a cAMP response element. It is likely that this site plays a role in maintaining BTK transcription in mature B cells.