The repair kinetics of DNA single- and double-strand breaks (SSBs, DSBs) induced with two carcinogenic epoxides, propylene oxide (PO) and epichlorohydrin (ECH), was studied in human diploid fibroblasts. The methods used were: alkaline DNA unwinding (ADU), the comet assay, and pulsed field gel electrophoresis (PFGE). About 70% of SSBs, measured by ADU, were rejoined after the treatment with 5 mMh and 10 mMh of PO within 20 hr, and the half-life was estimated to be approximately 15 hr. On the other hand, effective rejoining of SSBs after ECH treatment was observed only at a dose of 1 mMh (a half-life of approximately 15 hr), whereas after 2 mMh treatment, only 26% of SSBs could be rejoined within 20 hr. Furthermore, the use of the comet assay demonstrated that DNA strand breaks were effectively rejoined after PO and ECH treatment at doses of 5-10 mMh and 0.5-1 mMh, respectively. About 76% and 83% of DSBs induced by 5 and 10 mMh of PO, respectively, were rejoined within 4 hr after the treatment (a half-life of approximately 2.5 hr), with little further repair thereafter. At lower dose of ECH (1 mMh) a half-life for DSBs rejoining was estimated to be approximately 2 hr; however, only 29% of DSBs were rejoined within 2 hr at the higher dose of 2 mMh. After 18 hr, the rejoining following treatment with a lower dose was negligible. At a higher dose, a rapid accumulation of DSBs was observed, probably as the result of cell death and DNA degradation. The results demonstrate the capability of human diploid fibroblasts to repair DNA SSBs and DSBs at low-to-moderate doses of the epoxides. A weak capacity to rejoin DNA strand breaks induced by higher doses of ECH may be a consequence of its higher DNA alkylation activity and approximately 10 times higher toxicity compared to PO.