Sister chromatid exchanges (SCE) were studied in chromosomes of peripheral blood lymphocytes taken from 19 normal individuals of both sexes and cultured in the presence of 5-bromodeoxyuridine (BUdR). BUdR was added to the culture in final concentration of 15 or 30 microgram/ml for the last 28 hours of cultivation. The chromosome preparations were stained with the fluorescent dye Hoechst-33258, followed by Giemsa staining. At BUdR concentration of 15 microgram/ml the mean number of SCE was significantly less (8.9 per metaphase), that at the concentration of 30 microgram/ml (12.3 per metaphase). The incidence of SCE among 19 individuals varied from 6.5 to 13.7. The mean numbers of SCE in five individuals were significantly different from the mean number calculated for the whole group of 19 subjects. There were no differences between the sexes in the incidence of SCE. The distribution of SCE between chromosomes was found to be proportional to the chromosome length and to the amount of DNA. However, the chromosomes of groups E, F and G were highly significantly under-represented in the number of exchanges.