The basal levels of superoxide dismutase (SOD) activity and chromosome aberration (CA) and sister-chromatid exchange (SCE) frequencies were examined in cultured fibroblasts or Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs). These cells were derived from patients with chromosome instability syndromes (CISs) including Bloom's syndrome (BS), Fanconi's anemia (FA) and ataxia telangiectasia (AT). Embryonal fibroblasts and LCLs from normal subjects served as controls. Although LCLs tended to exhibit a higher SOD level than fibroblasts due to an elevation of Cu/Zn-SOD activity, BS and FA fibroblasts with increased frequencies of CAs and/or SCEs showed abnormally elevated SOD activity due to the manifold increase of Mn-SOD levels compared with control cells. However, BS and AT LCLs with almost control levels of CA and SCE frequencies showed no, or a slightly elevated, SOD activity, suggesting a possible selection of such cells during EBV transformation. The observed parallelism between the SOD activity and the cytogenetic manifestation may imply an involvement of active oxygen species, especially superoxide radicals, in the increased chromosome damage of CIS cells.