Considerable advances have been made during recent years, with regard to the detection and quantification of carcinogen- or mutagen-induced, structural modifications in the DNA of mammalian cells, by the introduction of immunoanalytical methods, in particular in conjunction with monoclonal antibodies (Mab). Antibodies are characterized by an outstanding capacity for the specific recognition of subtle alterations of molecular structure. They can, therefore, be used as sensitive detection probes in assays for DNA modifications caused by low levels of DNA-reactive (e.g., environmental) agents. Depending on the purpose of analysis, various types of immunoassays can be performed. The competitive radioimmunoassay (RIA) represents a routinely applicable, reproducible and sensitive assay for the detection of defined carcinogen-DNA adducts in hydrolysates of cellular DNA, in body fluids or in urine. Depending on their particular design, enzyme immunoassay (EIA) may have exceptionally low detection limits, due to the enzymatic amplification of the measured radioactivity or colour intensity. Similarly, recently established immuno-slot-blot (ISB) techniques are also characterized by very high sensitivity. Immunocytological assays (ICA) use Mab in conjunction with electronically intensified immunofluorescence for detection of modified DNA components in the nuclei of individual cells. Finally, single modified deoxynucleosides can be detected and localized in individual DNA molecules by immuno-electron microscopy (IEM).