The reaction of the antitumor antibiotic anthramycin with cellular DNA and the ability of normal human fibroblasts cells and xeroderma pigmentosum (XP) cells to respond to this injury has been evaluated. The binding of [15-3H]anthramycin to cellular DNA in human skin fibroblasts occurred in a linear manner up to 6 h. Treatment with unlabeled antibiotic resulted in unscheduled (repair) DNA synthesis in human skin fibroblasts maintained in hydroxyurea, whereas negligible unscheduled DNA synthesis was observed in cells of an excision-defective strain of XP. Confluent nondividing normal skin fibroblast cells were able to remove 86% of the bound anthramycin within 72 h, however XP cells were only able to remove 49% during the same incubation period. These results are discussed in terms of the types of DNA damage produced by anthramycin in vitro and the likely repair pathways involved in removing lesions produced on DNA by anthramycin.