Fanconi's anaemia (FA) is an autosomal recessive mutation associated with constitutional chromosome aberrations. Patient's cells show increased susceptibility to different mutagens, especially bifunctional alkylating agents. Asymptomatic heterozygotes have a population prevalence of 1 in 300, and have been considered at risk for cancers. It has also been shown that their cells are especially sensitive to some chemicals in vitro. Laboratory tests are presented and discussed which are adequate for FA heterozygotes identification and for understandings the basic defect of this mutation.