Prions are conformational isomers of normal proteins that lead to "transmissible spongiform encephalopies" in mammals and different phenotypes between isogenic cells in yeast. The [PSI+] prion in yeast has a pleotropic omnisuppressor phenotype that has been hypothesized to confer an evolutionary advantage; this was proposed as a possible explanation for the maintenance of prion-causing domains in the gene pool. It was predicted that the [PSI+] state in yeast cells would be associated with an overall increase in mutation rate relative to [psi-] cells. It was also predicted that there would be a significant increase in resistance to the growth inhibitors bleomycin and paraquat between yeast that are [psi-] and yeast that are [PSI+]. Mutation rate was measured by monitoring frequencies of back mutation at several loci by plating [PSI+j and [psi-] cells on omission media. Adaptation to growth inhibitor was measured in two ways: by colony survival after successive replication onto plates containing either paraquat or bleomycin at increasing concentrations, and by the cell density of liquid cultures containing inhibitor and either [PSI+] or [psi-] cells with concurrent spot plating of successive dilutions to estimate cell viability. No difference was noticed between the mutation rates of [PSI+] and [psi-] cells, while lack of sufficient trials prevented the formation of any conclusions relevant to the hypothesis for the adaptation rate data. Further experimentation is recommended.