Melanoma is a highly aggressive neoplastic disease attributed to transformed melanocytes. The efficacy of regimens of cytotoxic chemotherapy for advanced stage patients does not exceed 20%. Search for lymphocyte markers of patients' sensitivity to chemotherapy provides a rational basis for development of cytotoxic chemotherapy. Using blood lymphocytes we evaluated efficacy of BER and MMR, expression of MLH1, MSH2 and FasR, and cell death in melanoma patients relative to clinical response to chemotherapy. We found that LDCI-chemotherapy (lomustine, dacarbazine, cisplatin and interferon gamma), induced AP sites and DNA ss-breaks which repaired trough BER pathway. However, neither initial DNA damage nor the rate of their repair correlated with clinical response. This result prompts us to think that this type of damage is not crucial in cytotoxic effect of LDCI-regimen of chemotherapy. DNA ds-breakes appeared downstream ss-breakes were attributed to repair of 06-methylguanine by MMR mechanism in PHA-stimulated lymphocytes. The number of ds-breakes appeared by 48 correlated with positive clinical response of patients to chemotherapy. The same link was observed between clinical response and the number of dead lymphocytes. However, there was no correlation between clinical response and expression of MLHI + MSH2 and FasR. These results imply possible contribution of crosslink repair through NER pathway to formation of DNA ds-breaks as well as to cytotoxicity of LDCl-therapy. The observed link between high level of secondary ds-breaks and positive response to chemotherapy indicates the potential of these instruments to serve as prognostic end point in clinical trials.