We describe a new flow-system technique for high-speed quantitative karyotyping. Metaphase chromosomes are isolated from cells, stained with a DNA-specific fluorochrome, and individually measured for stain content at rates up to 10(5)/min in a flow microfluorometer. The resulting distribution contains quantitative information about the relative frequency of occurrence and the chromosomal DNA content of the measured chromosomes. Such analyses of Chinese hamster chromosomes are in good agreement with independent results from scanning cytophotometry and with cytogenetic observations of the banded chromosomes. Analyses of two karyotypically different clones of Chinese hamseter M3-1 lines provide the quantitative characterization of two stable translocations. Initial data on human chromosomes are presented and agree well with results of scanning cytophotometry and suggest potential clinical applications as preparative and instrumental techniques improve.