The effect of cyclophosphamide metabolites was studied on chromosomes of FAnconi anaemia patients, parents and controls. A high susceptibility of Fa patients chromosomes was observed when low concentrations of sera of a cyclophosphamide-treated patient was added to PHA-stimulated lymphocyte cultures. No effect was observed with comparable concentrations on cells from FA parents or controls. The high susceptibility of FA cells is discussed in relation to the high sensitivity of FA patients to cyclophosphamide when used as a conditioning drug for bone marrow graft.