In order to correlate the lipid composition of the membrane of Lactobacillus acidophilus CRL 640 with the freeze-thaw behaviour of the cultures grown at different temperatures, fatty acid methyl esters (FAMEs) from extracts grown at 25, 30, 37 and 40 degrees C were obtained and compared. Cultures grown at 25 degrees C (M25) exhibited more resistance to the freeze-thaw process probably because of an increase in C18:2 and C16:0 fatty acids. This culture also exhibited a lesser amount of phospholipids as shown by the sugar : phosphorus ratio. In all cases, the presence of the uncommon 10-hydroxyoctadecanoic acid was determined. From the extracts of the M25 and M37 cultures, diacylphosphatidylglycerol, cardiolipin, diglycosyldiglycerides, triglycosyldiglycerides and neutral lipids were isolated and identified. The structural elucidation was carried out by FAMEs and sugar analysis and by mass spectrometry using fast atom bombardment ionization. The changes in lipid composition due to different growth temperatures could be indicative of the resistance of the bacteria to freeze-thaw processes.