To identify the trans-acting factors involved in autonomously replicating sequence (ARS) function, we initiated a screen for Saccharomyces cerevisiae mutants capable of stabilizing a plasmid that contains a defective ARS element. The amm (altered minichromosome maintenance) mutations recovered in this screen defined at least four complementation groups. amm1, a mutation that has been studied in detail, gave rise to a 17-fold stabilization of one defective ARS1 plasmid over the level seen in wild-type cells. The mutation also affected the stability of at least one plasmid bearing a wild-type ARS element. amm1 is an allele of the previously identified TUP1 gene and exhibited the same pleiotropic phenotypes as other tup1 mutants. Plasmid maintenance was also affected in strains bearing a TUP1 gene disruption. Like the amm1 mutant, the tup1 disruption mutant exhibited ARS-specific plasmid stabilization; however, the ARS specificities of these two mutants differed. The recovery of second-site mutations that suppressed many of the tup1 phenotypes but not the increased plasmid maintenance demonstrates that the plasmid stability phenotype of tup1 mutants is not a consequence of the other defects caused by tup1.