From the same blood samples, blood cultures were set up to study a possible effect on die frequency of sister chromatid exchanges, another indicator for genotoxic effects.Here we report preliminary results which clearly demonstrate that strain above die aerobicanaerobic threshold induces DNA migration in human blood cells.The implications for the use of the SCG assay are discussed.To determine defined parameters of strain, a multiple step test was performed with stepwise increase in physical load, which included the following measurements: lactate concentration in peripheral blood activity in blood serum.CK was determined before the test, min after the end, as well as, and hlater.In a second test, which was performed week later, the same persons were studied after running for min at a fixed individual speed.The speed was determined according to the lactate measurements of the first run to ensure a metabolism under strictly aerobic conditions below the aerobicanaerobic threshold.Venous blood samples for the SCG assay were taken before the start, min after the end of the test, as well as, and hlater.Each blood sample was divided into two portions, coded, cooled with ice and brought to two different laboratories, where the SCG assays were run independently under the same experimental conditions.In one laboratory, DNA migration was measured as image length by using a calibrated scale in the eyepiece of the microscope, while the other group made measurements by image analysis. In these evaluations, the tail moment was determined.Then the coverslips were removed and a top layer of; J. LMPA was added and the slides were again kept cold for min.To electrophorese the DNA, an electric current of V and mA was applied for min.Alkali and electrophoresis treatments were performed in an ice bath.All of these steps were conducted under dim light to prevent the occurrence of additional DNA damage.The slides were allowed to sit for min and this step was repeated twice.It can also be seen that in person, who is an active sportsman, the baseline CK value is higher than in the other two test persons.In the second test, all subjects ran for min at individual speeds under strictly aerobic conditions.It is unclear whether this person, who is absolutely untrained, shows a small positive effect even under aerobic conditions or whether the lactate threshold is lower in this person.The small increase may also be due to normal variation.Further studies will have to work out timeeffect and doseeffect relationships in more detail.In contrast to the response in the SCG assay, physical activity did not lead to a significant effect in the SCE test. SCE are a sensitive cytogenetic indicator for DNA damage, respond to a variety of DNA lesions and are widely used in biomonitoring. The conflicting results may be explained by the different sensitivities of the two tests for different kinds of lesions.Thus the SCG assay has been shown to detect the DNA strand breaks induced by ionizing radiation, radiomimetic chemicals and oxygen radicals with high sensitivity while such breaks do not efficiently induce SCE. The cause of DNA migration in the SCG assay after strong physical activity is not clear but oxygen damage may be involved.