UNLABELLED: 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. Mg homeostasis is important in Salmonella and has been reported to play a critical role in their thermotolerance. However, the role of Mg in thermotolerance of other bacteria, in particular probiotic bacteria, still remains a hypothesis. In this study, the effect of Mg 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 Zn and Na . Concentrations of Mg between 10 and 50 mmol l were found to increase the bacterial survival upon heat challenge. Remarkably, Mg addition at 20 mmol l led to a 100-fold higher survival of L. rhamnosus GG upon heat challenge. This preliminary study also showed that Mg shortened the heat-induced extended lag time of bacteria, which indicated the improvement in bacterial recovery from thermal injury. SIGNIFICANCE AND IMPACT OF THE STUDY: In order to improve the productivity and stability of live probiotics, extensive investigations have been carried out to improve thermotolerance of probiotics. However, most of these studies focused on the effects of carbohydrates, proteins or amino acids. The roles of inorganic salts in various food materials, which have rarely been reported, should be considered when incorporating probiotics into these foods. In this study, Mg was found to play a significant role in the thermotolerance of probiotic lactobacilli. A novel strategy may be available in the near future by employing magnesium salts as protective agents of probiotics during manufacturing process.