A single haematological alteration is not sufficient to diagnose whether it is a radiation-induced change or not. For the differential diagnosis of possibly radiation-induced changes in the peripheral blood and blood-forming organs, information on the radiation exposure in terms of time, quality, quantity and localization, and the clinical symptoms have to be taken into account. Ionizing radiation within the dosage range considered here produces cell division delay, mitotic inhibition, chromosomal damage or interphase cell death; it thereby interferes with the steady-state equilibria in the cell-renewal systems of the organism (Bond et al., 1965; Little, 1968). The cause of haematological changes appearing immediately after a short-term, external whole-body radiation exposure has been described and analysed elsewhere in this Manual. The critical cell component is the 'stem cell compartment' which is highly radiosensitive and suffers damage but, because stem cells cannot be identified morphologically, a direct study of stem cell injury is not possible.