Abstract: In order to obtain more insights into the molecular mechanisms of action of polyene antibiotics in yeast cells, a number of nystatin sensitive mutants of Saccharomyces cerevisiae were isolated by ethylmethane sulfonate mutagenesis. The mutants were characterized in terms of their sensitivity test to stresses such as high temperature or high concentration of monovalent and divalent cations. The invertase overlay assay method suggests that some of these mutants are defective for vacuolar protein sorting. By tetrad analysis four of these mutants were found to possesses single gene mutation.