Eosinophils as well as neutrophils, monocytes and B lymphocytes are noted for lacking normal cytochrome b 558 in patients with X-linked chronic granulomatous disease. The eosinophils of an X-linked male patient, however, fully expressed surface cytochrome b 558, generated superoxide anion to a normal extent and definitely expressed the large subunit of cytochrome b 558 (gp91- phox). His mononuclear leukocytes contained a diminished amount of gp91- phox mRNA with normal coding sequences. All the coding sequences and a putative poly (A) signal of his gp91- phox gene were normal. These results indicate that eosinophils have a specific mechanism to express gp91- phox and suggest that the mechanism lies at the transcriptional step of the gene.