The radiation sensitive gene rad 21 of Schizosaccharomyces pombe is involved in the repair of double-stranded breaks in DNA and is essential for mitotic growth. The hHR21sp gene is its human homologue. In an attempt to investigate the role of hHR21sp in DNA repair, we studied the effects of UV and gamma-ray irradiation on hHR21sp gene expression in normal human peripheral blood cells, and non-iradiated peripheral and bone marrow cells from Fanconi anemia (FA) patients who have shown DNA repair deficiency. Total steady state RNA was extracted from peripheral blood cells and bone marrow. RNA transcripts were quantified after RT-PCR and Southern blot, phosphoimmage and autoradiogram analysis. The results were compared with control groups. hHR21sp expression was significantly increased from 3 h to 9 h after UV irradiation in peripheral blood cells from normal subjects at doses of 40-80 j/m2 (P < 0.05). hHR21sp was also up-regulated by gamma-ray irradiation at 6 h to 9 h at dose of 1 to 5 Gy (P < 0.01), which was more significant than the UV irradiation. In the non-irradiated FA patient group, hHR21sp expression was decreased in bone marrow hematopoietic cells (P < 0.05). After activation by PHA and IL-2, there was still a significant depression in expression by the FA patients peripheral blood cells compared with control groups (P < 0.05). hHR21sp was up-regulated at doses and times irradiated at the range tested in normal peripheral blood cells, and is more affected by gamma-ray irradiation than UV irradiation. FA patient bone marrow hematopoietic cells and peripheral blood mononuclear cells showed down-regulation of hHR21sp expression. The results imply that defects in DNA repair via hHR21sp expression may play an important role in the pathogenesis of FA syndrome.