In a study of the expression of folate-sensitive fragile sites in five normal individuals using RPMI medium containing methotrexate (MTX), we observed a high frequency of "sister chromatid intercrossings" (SCI) that is, the intersection of sister chromatids. The location of SCIs corresponded to fragile sites in 54.2% of the cases. Of the SCIs observed in each individual, 43%-53% were located at the same bands as their expressed fragile sites. Furthermore when RPMI+MTX medium was used instead of F-10 medium, the incidence of SCIs increased tenfold. We suggest that SCIs could indicate the existence of a pre-lesion. © 1988 Springer-Verlag.