The single-cell gel electrophoresis (or comet) assay is a rapid and sensitive fluorescence microscopic method that is used for the detection of DNA damage at the individual cell level. Comet assay under neutral conditions allows the detection of DNA double-strand breaks, considered to be the biologically relevant radiation-induced lesion. In this report we describe modifications of the neutral comet method and its use for studying DNA damage and its repair in irradiated human lymphocytes.