We have used a set of deletion mutations in the ARS1 element of Saccharomyces cerevisiae to measure their effect on chromosome stability. This work establishes the previously proposed existence of three domains in ARS1. Domain C, which we had previously inferred, but not proved, to be a part of ARS1, is now established. In addition, we show that increasingly large deletions of the domain have increasingly large effects, which was not realized before. Furthermore, we have provided the first positive evidence for the central importance of a 14-base-pair core sequence containing the ARS consensus element by showing that it has the ability to act as a replicator on a plasmid containing no other ARS1 flanking sequence. The method of analyzing plasmid stability used in our study employs a novel and sensitive flow cytometry assay for β-galactosidase. We discuss ways in which flow cytometry, based on this assay, could be generalized beyond its particular application in this work to studying other aspects of the cell biology of yeast and higher cells. The actual flow cytometry method will be described in detail elsewhere.