The inactive fragile X chromosomes of a 47,fra(X),fra(X),Y male with a typical fragile X phenotype were successfully separated from the active homologues by means of somatic cell hybridization. It was shown by FUdR-induction and caffein-posttreatment that the separated inactive X chromosomes expressed their fragile sites and that the presence of an active mutated X chromosome was not a prerequisite for fragile X expression. The fragility seems to be an intrinsic property of the individual fragile site. This result is in favour of the classical concept that the fragile site at Xq27.3 has a primary pathogenetic function in this syndrome, although the fragility itself could represent a secondary phenomenon related to an unknown alteration of the DNA in this chromosome region. It is also concluded that inactivation of the fragile X chromosome in females is not responsible for either false negative fragile X findings or the observation of fragile X negative colonies isolated from fragile X positive fibroblasts in heterozygotes.