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An action spectrum for the induction of pyrimidine dimers in human peripheral lymphocytes was determined between 254 and 405 nm. The presence of pyrimidine dimers was determined as UV-induced lesions that were sensitive to the dimer-specific endonuclease from Micrococcus luteus in conjunction with agarose gel electrophoresis. The rate of induction of pyrimidine dimers was maximal at 254 nm. These values can be compared with action spectra for UV-induced in vitro responses of lymphocytes.

Excision repair kinetics of UV-induced pyrimidine dimers in DNA of phytohemagglutinin (PHA)-stimulated human peripheral blood lymphocytes were compared to unstimulated lymphocytes using a dimer-specific endonuclease from Micrococcus luteus in conjunction with agarose gel electrophoresis. Removal of pyrimidine dimers could be detected within 6 h after irradiation only in PHA-stimulated lymphocytes. However, incorporation of [3H]thymidine as UV-induced unscheduled DNA synthesis was detected in the unstimulated lymphocytes in the 6-h period. The number of pyrimidine dimers remaining in unstimulated lymphocytes was approximately 85% after 24 h as compared to less than 25% in stimulated cells.