In the presence of polyethylene glycol (PEG), budding cells of Saccharomyces cerevisiae in the early log phase were transformed by exogeneous plasmid DNA without additional specific chemical or physical treatments. This capacity of the yeast cells to become competent was strictly dependent on the growth phase, being induced in the early log phase, becoming maximum between the early and mid log phases, and then disappearing rapidly in the mid log phase. The transformation was most efficient at pH 6, and the frequency increased with increasing DNA and cell concentrations. PEGs with average molecular sizes between 1000 and 3500 showed almost the same effects, and were used most efficiently at 35%. The transformation frequency of S. cerevisiae was markedly enhanced when the oxidized form of glutathione (GSSG), but not the reduced form, was included in the mixture comprising early log phase cells, plasmid DNA, and PEG, and the transformation system with GSSG could be used as a convenient transformation method for the yeast S. cerevisiae.