The leu2 revertant content of a Saccharomyces cerevisiae cell culture increases as the leucine concentration in the nutrient solid medium decreases. Reversions form in the S-phase of the cell cycle. If a cell culture from a medium with a low concentration of leucine containing the revertants which have just formed is transferred on a medium with a normal or higher than normal leucine content, these 'newborn' revertants disappear at the end of the G1-phase or at the beginning of the S-phase of the next cell cycle. These data can be explained either by a difference in the ability of revertants formed in the culture to compete with the cells of the initial strain on different media, or on the basis of the intermediate heteroduplex model proposed by F.W. Stahl (1988).