The numbers of melanocytic nevi, localized benign proliferations of melanocytes, have been shown to be associated with an increased risk for development of melanoma. In the present study we have developed an alternative post-labelling method for determination of levels of cyclobutane thymidine dimers (T=T) as dinucleotides at sensitivities sufficient for analysis of human skin samples. Using the developed method, the induction of T=T was determined in melanocytic nevi in situ and surrounding skin, obtained from seven subjects, after exposure to solar simulating radiation. The T=T level in nevi was found to be 1- to 4.5-fold lower than that in surrounding skin and the difference was statistically significant (Student's t-test, P < 0.05).