SUMMARY: Growth of a strain of Saccharomyces cerevisiae in an inositol-deficient chemically defined medium resulted in marked changes in the composition of the cell wall. Organisms grown in the complete absence of inositol possessed a weakened cell wall which contained more glucan and hexosamine but less mannan, phosphorus and protein than normal walls. The amino acid contents of the two types of cell wall were quite different but no significant change occurred in either the amount or the fatty acid composition of cell wall lipid. Treatment with ethylenediamine facilitated the isolation of three fractions, A, B, and C from normal and from deficient cell walls. The mannan and protein content of fractions A were strikingly similar. Deficient fractions B and C however contained considerably less mannan and protein but more glucan and hexosamine than the corresponding normal fractions.