[Decrease in the spontaneous frequency level of sister chromatid exchanges in cell division in culture].

The spontaneous level of sister chromatide exchanges (SCE) registered in human lymphocytes is shown to depend on the moment of BUdR introduction and the time of fixation. In early periods of BUdR introduction and fixation the general spontaneous level of SCE may be observed and in later periods only that part of SCE may be registered which is caused by internal conditions. The difference between the first and second results makes the part of SCE conditioned by the environmental effects.