A process is described for the production of an immunostimulant by submerged cultivation of Ganoderma lucidum in which mycelium from agar plates or a fermentation broth is added to a liquid medium in a shake flask or a bioreactor containing nutrients such as malt extract, yeast extract, peptone and glucose having access to air or to which air is added, and which is kept in constant movement at approx. 28° C. At the proper conditions, there will be an increase in the production of extracellular lentinan, which is shown to be a better immunostimulant than intracellular lentinan. The extracellular product is precipitated from the growth medium by means of methods for the precipitation of microbial polysaccharide.