Evaluation of chromatid aberrations induced in culture by DNA cross linking agents provides the most reliable method currently available for the diagnosis and exclusion of Fanconi's anaemia. However, at appropriate concentrations of clastogenic agent the aberration frequency in an unaffected subject may be very low and thus it may be difficult to confirm that the treatment was effective. Data are presented to show that sister chromatid exchange analysis can be used to monitor the effectiveness of the clastogen treatment and thereby increase the reliability and efficiency of the assay.