Micronuclei can be measured through a conventional method after staining with Giemsa or fluorescence dyes for DNA. However, a technique with cell proliferation control should be preferred. This is done by incubation with cytochalasin B and counting the micronuclei in binucleated cells. Satisfactory dose relationships are observed after irradiation of human lymphocytes in vitro. The RBE for fast neutrons is around three. An automatic analysis is possible by image analysis. The dose range in which significant increases can be observed is 0.3 to 5 Gy X-rays. The assay becomes more sensitive when the micronuclei are determined only in B-lymphocytes. Another possibility exists by determination of the number of micronuclei with centromeres. For this purpose the hybridization with pancentromeric DNA probes and fluorescence labelling is of advantage. By this technique a radiation dose of 0.1 Gy X-rays can be detected. It is apparently also possible under these conditions to detect radiation exposures which have taken place decades before the measurements.