A growth enhancer for Lactobacillus murinus, Lactobacillus salivarius, or a bacterium that belongs to the genus Lactobacillus and that has a homology of 16S rDNA of 90% or higher with Lactobacillus murinus or Lactobacillus salivarius includes a β-glucan having a molecular weight of from 0.2 K to 50 K. A regulatory T-cell number increasing agent, a method of enhancing growth of a lactic acid bacterium, a method of increasing the number of regulatory T-cells, a method of evaluating a regulatory T-cell number increasing effect, and a method of evaluating a growth enhancing effect on a lactic acid bacterium are also provided.
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