Genetic transformation of intact cells of Saccharomyces cerevisiae, achieved by incubating the cells with plasmid DNA in the presence of PEG, could be enhanced, not only by pretreatment of the cells with Li+ and 2-mercaptoethanol, as has been reported previously, but also by pretreatment with proteolytic enzymes. The efficiency of transformation with 2-mercaptoethanol rose dramatically when the pretreated cells had been handled in osmotically stabilized media. Following all the pretreatments the cells became leaky for nucleic acids as detected by the presence of endogenous RNAs in the medium. The pretreatments evidently facilitated the passage of transforming DNA across the cell wall.