The fragile X mutation is the result of an abnormal expansion of a CGG repeat sequence in the FMR-1 gene. Molecular techniques enable the detection of the mutation and also of the exact length of this DNA sequence, allowing the classification of the tested subjects as normal, carrier or affected. We propose a protocol of analysis that combines a method of non-radioactive PCR, Southern blotting and cytogenetic testing. This protocol can be used for screening programme of selected groups of mentally retarded individuals and for prevention studies in families at risk.