The current method for scoring micronuclei as a measure of genetic damage rate in peripheral blood cells is to enumerate this end point in cytokinesis-blocked binucleated cultured lymphocytes. However, one can expect that, due to chronic exposure to genotoxins or inherent genetic instability, micronuclei may be expressed continually in vivo in dividing cell populations such as the progenitor cell lineages leading to mature lymphocytes or erythrocytes. Consequently, micronuclei may already be expressed in peripheral blood lymphocytes prior to culture. In view of these considerations, we have performed a study in children living in regions of Belarus that are contaminated by radionuclides from the Chernobyl disaster and compared their micronucleus frequency in erythrocytes, nondivided lymphocytes, and cultured cytokinesis-blocked binucleated lymphocytes to that of controls living in noncontaminated areas. Preliminary data presented in this paper indicate a significant two- to fourfold increase in micronucleus expression (P < 0.05) in exposed children relative to controls in erythrocytes or peripheral blood lymphocytes in blood smears as well as in mononuclear and cytokinesis-blocked binucleated lymphocytes in cultures. The measurement of micronuclei in nondivided mononuclear lymphocytes represents chromosomal damage expressed during in vivo divisions. The micronuclei in binucleated cultured cells represent micronuclei expressed ex-vivo and may include micronuclei already present in a cell prior to tissue culture. These preliminary data suggest that a different spectrum and level of damage may be observed in nondivided mononuclear lymphocytes, binucleated lymphocytes, and erythrocytes and that a combination of these approaches may provide a more comprehensive assessment of the extent of genetic damage induced by chronic exposure to radionuclides or other genotoxins in haematopoietic tissue.