The Ψ+ extrachromosomal determinant in the yeast Saccharomyces cerevisiae suppresses certain UAA markers and increases the efficiency of suppression of UAA suppressors and certain frameshift suppressors. Although the exact nature of Ψ+ determinant is unknown, it is believed to be a self-replicating cytoplasmic factor affecting some component of the translational machinery. In this report we describe growth conditions for efficient mutation or elimination of the Ψ+ determinant. Incubation of Ψ+ cultures in hypertonic nutrient medium resulted in rapid conversion to a culture containing predominantly Ψ- cells during the growth cycle. The kinetics of Ψ+ to Ψ- conversion established that the occurrence of Ψ- cells was due to induction and not to selection of pre-existing Ψ- cells. The results suggest that the replication of the Ψ+ determinant is sensitive to hypertonic conditions.