Skin melanoma is believed to arise from the malignant development of congenital or acquired pigmented nevi under the action of various causative agents. A study of samples of skin melanoma tissues, as well as pigmented nevus tissues that were obtained from patients of both sexes, was carried out using enzyme-linked immunosorbent assay. The study included a comparative analysis of activities of plasmin and plasminogen, as well as the activities and concentrations of urokinase type plasminogen activator, tissue-type plasminogen activator, and an inhibitor of plasminogen activator, as well as the levels of a number of growth factors, including vascular endothelial growth factor and its receptor, epidermal growth factor and its receptor, transforming growth factor, fibroblast growth factor, and finally insulin-like growth factors 1 and 2. The data underpin the possibility of malignant transformation of skin nevi and indicate provisional mechanisms of tumorigenesis. These results are useful for the development of preventive and risk evaluation methods for neoplastic transformation.