The nib 1 allele of yeast confers a sensitivity to an endogenous plasmid, 2 mu DNA, in that nib 1 strains bearing 2 mu DNA (cir+) exhibit a reduction in division potential. In the present study, the reduction in division potential characteristic of nib 1 cir+ strains is shown to be dependent on the simultaneous presence of both the A and the D open reading frames of 2 mu DNA as well as on the presence of an unidentified extrachromosomal element other than 2 mu DNA. Furthermore, in nib 1 strains, an uncharacterized extrachromosomal element can cause a less severe reduction of division potential in the absence of intact 2 mu DNA. Thus, the nib 1 allele may confer a generalized sensitivity to extrachromosomal elements.