Repeated cultures from 5 healthy donors were set up in 3 tissue-culture media: TC109, TC199 and TC1640. For each donor, 3 blood samples were taken at 1-week intervals, making a total of 5 x 3 x 3 = 45 cultures. Our results show a large variability in the individual SCE base-line frequency. This variability can be partly attributed to the different tissue-culture media or to the donors themselves. Our results, however, stress the importance of the proliferating rate of the culture in determining the SCE frequency.