Lymhocytes from clinically healthy Holstein animals (8 males and 23 females) were cultured in media containing 5-bromodeoxyuridine (10 microgram/ml) for two cycles of deoxyribonucleic acid replication (48 h). The exchange of sister chromatids per cell varied from 1 to 16 with a mean of 5.4 and a standard deviation of 2.1 in 603 differentially stained metaphase chromosome spreads. The major fraction of exchanges in the X chromosome were located in the region of the subcentromeric G negative bands q1 and q2. When the cells were pulse labeled with tritium labeled thymidine (1 muCi/ml) for the last 6 h of the cell cycle, the rate of exchange was higher in the late replicating X chromosome (facultative heterochromatin) in comparison to its homologue (active X). This study characterizes the yield of sister chromatid exchange in lymphocytes exposed to a given amount of 5-bromodeoxyuridine in a population of normal individuals so that it may be used as a standard for 1) a diagnostic tool in pathological conditions and 2) as an assay of chromosome stability in relation to environmental hazards.