The aim of this work was to demonstrate the presence of intact chromosomes in micronuclei. Such evidence was based on the re-expression of metaphase chromosomes by micronuclei fused with whole cells. Three different experimental approaches were used; they differed only in the criteria used to discriminate between the chromosomes of micronuclear origin and those present in the cell to which micronuclei were fused. These methods involved the expression of: (a) human chromosomes in mouse cells, (b) radioactively labelled human chromosomes in unlabelled human lymphocytes and (c) Chinese hamster chromosomes in Chinese hamster cells with differentially stained chromosomes. With all methods, evidence for the presence of one or few chromosomes in micronuclei was obtained.