The expression of folate sensitive common fragile sites was investigated in 82 normal healthy males and females of various ages. In 100 studied metaphases of each of these controls, between 0 and 56 lesions were detected (mean 18.3 +/- 10.3 SD). No significant difference was found between the mean number of expressed lesions in females and males. No age-effects were observed. Two "new" common fragile sites were discovered at 6p21 and 17q21. Their fragile site status, however, needs to be confirmed.