Whole chromosomes within micronuclei (MN) in cytokinesis-blocked human lymphocytes were detected by using anti-kinetochore antibodies obtained from the serum of sclero-derma patients. The primary antibody was localized using a peroxidase-labelied second antibody followed by a nickel chloride modification of the diamino-benzidine reaction to give a permanent slide preparation. Between 82 and 92% of colchicine-induced MN were shown to be kinetochore-positive. Results for spontaneously occurring micronuclei in young (20-35 years) and elderly (>65 years) subjects indicated that 42(±6) and 50(±6)%, respectively, contained kinetochores. Perhaps the more novel observation was that ∼12% of X-ray-induced micronuclei were kinetochore-positive and thus could have been the result of whole chromosome loss events. Kinetochore-detection in micronuclei provides a new approach to measure chromosome loss and therefore may be important in identifying aneuploidy-inducing agents.