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The incidence of sister chromatid exchanges during the second and third mitoses in Jungar hamster bone marrow cells and human lymphocytes is assessed at different times after mutagenic exposure to thiophosphamide. In the control, the mean number of sister chromatid exchanges in bone marrow cells did not change during three consecutive cycles irrespective of the rate of cell proliferation. Thiophosphamide exposure resulted in replicative or nonreplicative repair of injuries inducing sister chromatid exchanges. About 50-70% of injuries are eliminated during a replicative (mitotic) cycle. Nonreplicative repair is intensive in proliferating bone marrow cells and weakly expressed in blood lymphocytes. Key Words: sister chromatid exchanges; lymphocytes; bone marrow cells; thiophosphamideSister chromatid exchanges (SCE) are formed during DNA replication as realization of primary chromosome injuries. The number of such injuries and, therefore, SCE after mutagenic exposure gradually decreases due to reparative processes and death of cells damaged worst of all [3,6,9]. SCE-inducing injuries can be repaired in proliferating ceils during DNA replication and other phases of mitotic cycle and in nonproliferating cells [1,7,9]. Published data about the contribution of each of these processes to the decrease in the SCE level after mutagenic exposure are contradictory. However, it is the crucial point determining the conditions of SCE test for analysis of mutagenic loading in genetic monitoring.Our purpose was to assess the incidence of SCE during the second and third mitoses at different times after mutagenic exposure of Jungar hamster MATERIALS AND METHODSIn vivo experiments were carried out on 36 Jungar hamsters weighing 35 g. A 50-mg tablet of 5-bromo-2'deoxyuridine (BDU, Sigma), 80% of which was coated with an MK-6 biological glue for decelerating the drug absorption and prolonging its effect, was subcutaneously implanted into each animal. In parallel with this, experimental hamsters were intraperitoneally injected with thiophosphamide (TP) in a dose of 1.5 lag/g dissolved in normal saline. Controls were injected with saline alone. The animals were sacrificed by cervical dislocation after 13-37 h at 6-h intervals. Two hours before sacrifice the animals were injected eolchicine in a dose of 2 ~tg/g intraperitoneally. Bone marrow was collected from femoral bones.For in vitro experiments, human donor blood was incubated for 96 h in the presence of BDU. Blood specimens were treated with TP for 1 h (10

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