We have used a monoclonal antibody specific for UV-induced 6-4 photoproducts in an ELISA assay to determine the kinetics of loss of antigenicity from the DNA of lymphocytes obtained from four groups of people; normal controls and cancer patients who had either received chemotherapy, hormone therapy or no treatment at all. This result was confirmed on a matched pairs analysis of 12 breast cancer patients sampled before and after chemotherapy. We conclude, that chemotherapeutic treatment with alkylating agents modulate the capacity of UV-induced DNA-repair in human lymphocytes in a yet unknown way.