Ataxia telangiectasia (AT) is an autosomal recessive disease characterized by a progressive neuronal degeneration, immunodeficiency, cancer proneness and an extreme sensitivity to ionizing radiation. In this work, micronucleus dose-response curves for lymphocytes of normal and AT individuals, exposed in G(zero) to low LET gamma-rays and high LET fast neutrons, are compared. After gamma-irradiation, the micronucleus yields for AT lymphocytes are strongly increased compared with controls. The micronucleus dose-response curve for AT cells shows a linear dependence instead of a linear-quadratic one which is found for normal cells. After neutron irradiation, the increase in micronucleus yield above controls is less pronounced than with gamma-rays and the micronucleus dose-response curves are linear, as expected. The high increase in micronucleus yield compared with controls after gamma-irradiation further suggests the application of the micronucleus assay as a diagnostic tool for ataxia telangiectasia.