We have explored the induction of DNA repair synthesis in monocyte/B- and T-lymphocyte enriched cell fractions from 12 different human mononuclear blood cell populations. Unscheduled DNA synthesis was measured in monocyte/B- and T-cells after exposure to the DNA-damaging agents dimethylsulfate (DMS) and N-acetoxy-N-acetylaminofluorene in vitro. Also, the binding of DMS to DNA was measured. An increased DNA repair synthesis was measured in monocyte/B-lymphocytes after induction of the two different types of DNA lesions, whereas no induction of unscheduled DNA synthesis was observed in T-lymphocytes. A significantly higher DMS-DNA binding was also observed in monocyte/B-lymphocytes when compared with T-lymphocytes. Specific characterization of mononuclear blood cell populations used in biomonitoring of DNA adducts and repair is recommended.