Halothane (CAS 151-67-7) induced DNA strand breaks in isolated lymphocytes of two patients with a deficient DNA repair (xeroderma pigmentosum). In lymphocytes (resting cells) of healthy human donors and in L 5178 Y cells (proliferating cells) of mouse lymphoma, halothane did not induce demonstrable DNA strand breaks. The cells were exposed to 1.0 vol/% halothane for 60 min, and the DNA strand breaks were demonstrated by alkaline elution. The results suggest a possible genotoxic side effect of halothane in patients with deficiency in DNA repair.