A brief exposure of cells to Colcemid induces chromosomal nondisjunction. Making use of this behavior, we isolated several trisomic as well as monosomic cell clones from a cloned pseudodiploid Chinese hamster cell line. Karyotype analyses of these clones demonstrated that mainly small chromosomes such as Nos. 9 and 10 (or No. 11) were involved in chromosomal addition or loss. Nondisjunction was found to occur randomly in every member of the complement, but progeny cells possessing unbalanced chromosome constitutions, such as aneusomies of large chromosomes, appeared to die off or undergo karyotype remodeling by the time of the third or fourth mitosis after induction of nondisjunction.