The mechanism of inactivation of freeze-dried Lactobacillus bulgaricus during storage in maltodextrin under controlled humidity was investigated. Evidence is presented supporting the hypothesis that membrane damage occurs during storage. A study ore the lipid composition of the cells by gas chromatography showed a decrease in the unsaturated and saturated fatty acid content of the cell. Further evidence indicating membrane damage includes a decrease in membrane bound proton-translocating ATPase activity.