The viability of lyophilized cultures of Lactobacillus bulgaricus in skim milk, during storage at different temperatures, relative humidities, and atmospheres was investigated. Survival was greatest at 11% relative humidity and at 5 degrees C. Indirect and direct evidence is presented supporting the hypothesis that membrane damage occurs during storage. Experiments on the lipid composition of the cell membrane demonstrate that changes occur with time that are probably the result of oxidation. A study on the li!pid composition of the cell membrane by gas chromatography showed that the unsaturated/saturated fatty acid index changes with time during storage.