SUMMARY Mutants of Saccharomyces cerevisiae have been selected for resistance to the polyene antibiotics, etruscomycin, filipin, nystatin, pimaricin and rimocidin. All mutants are resistant to nystatin and there are several patterns of cross-resistance. The mutants were allocated to four unlinked genes pol I, 2, 3 and 5. A fifth gene, pol 4, was recovered as a double mutant with a strain carrying the pol I muta- tion. The pol 4 mutation does not cause resistance. pol I and pol 3 are allelic to the previously described nys I and nys 3 (Ahmed & Woods, 1967). There are correlations between the polyene used for mutant isolation and (i) the extent of cross-resistance; (ii) the selection of mutants at particular pol genes. Ultraviolet absorption spectra of non-saponifiable material extracted from representative mutants showed that all five genes affect the sterol composition of the cell. The major sterols found inpol+ are ergosterol and 24,(28)dehydroergosterol, the latter is not found in any of the mutants whilst ergosterol is lacking in pol z and only present at very low levels in pol 3. The spectra of extracts of pol I and pol 3 indicate the presence of new sterols (Woods, 1971). Mutants of pol I excrete sterol into the growth medium. Studies on double mutants indicate epistasis between pol genes with respect to sterol pattern and suggest that they are metabolically related in the sequence pol z +pol 3 +pol 5 +pol I +pol 4 +pol+. A rapid qualitative technique for the preparation of non-saponifiable extracts of yeast for sterol analysis is described.