Food-related carbohydrates and proteins are often used as thermoprotectants for probiotic lactobacilli during industrial production and processing. However, the effect of inorganic salts is rarely reported. Magnesium is the second-most abundant cation in bacteria, and commonly found in various foods. Mg2+ homeostasis is important in Salmonella and has been reported to play a critical role in their thermotolerance. However, the role of Mg2+ in thermotolerance of other bacteria, in particular probiotic bacteria, still remains a hypothesis. In this study, the effect of Mg2+ on thermotolerance of probiotic lactobacilli was investigated in three well-documented probiotic strains, Lactobacillus rhamnosus GG, Lactobacillus casei Zhang and Lactobacillus plantarum P-8, in comparison with Zn2+ and Na+. Concentrations of Mg2+ between 10 and 50 mmol l(-1) were found to increase the bacterial survival upon heat challenge. Remarkably, Mg2+ addition at 20 mmol l(-1) led to a 100-fold higher survival of L. rhamnosus GG upon heat challenge. This preliminary study also showed that Mg2+ shortened the heat-induced extended lag time of bacteria, which indicated the improvement in bacterial recovery from thermal injury.