A quantitative analysis of Colcemid-induced chromosomal nondisjunction in cultured Chinese hamster cells is presented. Chromosome counts were made immediately following nondisjunction and also determined for colonies arising after treatment. Nondisjunction of a single chromosome occurred randomly. When two or more chromosomes were involved, the process was nonrandom and appeared to be independent of chromosome size. The frequency distribution of the extra chromosomes in single and multiple trisomic clones which survived Colcemid treatment was nonrandom, although every group could be involved. Monosomy appeared in 0.8 % of the surviving clones. Although monosomy for each chromosome grouping was observed immediately after nondisjunction, only monosomics involving the smaller chromosomes proliferated beyond this stage to form a colony.