Sister chromatid exchanges were studied in cultured lymphocytes from ten normal healthy females.5-bromodeoxyuridine was added at a final concentration of 30 ug/ml at the beginning of the culture time. Hoechst-Giemsa technique was used in the chromosomal preparations. The mean of SCE frequency was 4.01/metaphase +/- 1.34 and a range of 0-18. The present study revealed the predominance of exchanges between sister chromatids among chromosomes of groups A, B and C