DNA double-strand breaks (DSB) may arise either spontaneously during cellular processes or as a result of exposure to DNA-damaging agents such as ionizing radiation, or radiomimetic agents such as restriction endonucleases or bleomycin. It is widely accepted that nonrepaired or misrepaired DSB are the main lesions leading to the production of chromosomal aberrations, mutagenesis, onco-genic transformation, and cell killing. Studies fo-cusing on this relationship, as well as the possible modulation of DNA repair mechanisms, are currently of major interest. A wide variety of test systems are available to study DNA damage. In the last few years, single-cell gel electrophoresis, commonly known as "comet assay," has been considered a rapid, sensitive, and visual method for quantifying DNA strand breaks and alkali-labile damage in individual cells. In this study, making use of the comet assay, we tried to find out if under conditions that maintain chromatin structure the DNA ligase from T4 phage is able to facilitate the rejoining of strand breaks with different end structures , induced by the restriction endonuclease MspI or bleomycin in living human lymphocytes in a nonproliferating state. T4 DNA ligase, as well as the restriction endonuclease or bleomycin, were introduced together by electroporation into human lymphocytes. Our results support the idea that it is possible to modulate the DSB-rejoining of different DNA strand-breaking agents by exogenous T4 DNA ligase. Environ. Mol. Mutagen. 32: 336-343, 1998