A method of sedimentation in alkaline sucrose gradients was used to study repair of gamma-induced DNA single strand breaks (SSB) and DNA degradation in HeLa cells treated with novobiocin (Nb), an inhibitor of topoisomerase II. After irradiation in a dose of 150 Gr, Nb in a concentration of 1 mM does not affect the effectivity of SSB repair and DNA molecular mass in the irradiated cells treated with Nb for 60-180 min before irradiation. Besides, it does not lead to additional DNA degradation in cells treated with Nb for 60-180 min before gamma-rays, as well as following a postirradiation incubation during 60-180 min. Nb in a concentration of 4 mM, much exceeding the Nb concentration when DNA synthesis and cell transit through the cycle are inhibited, causes the following changes. It does not affect DNA molecular mass in non-irradiated cells, inhibits repair of DNA SSB, causes partial DNA degradation, if cells are treated for 60-180 min before gamma-rays and during the following postirradiation incubation (60-180 min). Taking into account the Nb-mediated DNA degradation, the inhibition of DNA repair by Nb appears not significant. Since in a concentration, which inhibits topoisomerase II, Nb does not affect repair of gamma-induced DNA SSB, one may assume the lack of involvement of topoisomerase II into repair of these DNA lesions. Inhibition of DNA repair by 4 mM Nb may result from its effect on the number of proteins, including reparative DNA polymerase, rather than from a selective effect on topoisomerase II.