**1. Introduction**

Mushrooms, their fruiting bodies and mycelia have served as food and food supplements around the world. They are relatively less toxic and are rich in bioactive compounds, such as polysaccharides, proteins, minerals, and other nutrients [1]. Beneficial activities associated with mushroom fruiting bodies and mycelia include antitumor [2], antimutagenic [3], antiviral [4], and antioxidant activities [5]. Some mushrooms alleviate the risk of diseases, such as Parkinson's and Alzheimer's disease, and hypertension [6].

Mushroom mycelia contain bioactive compounds as well as mushroom fruiting bodies, which have been investigated for their medicinal value. For example, oral administration of *Sparassis crispa* mycelia resulted in antitumor responses in tumor-bearing ICR mice [7]. The ability of erinacines, the bioactive compounds of *H. erinaceum* mycelia, to promote nerve health has been documented [8]. However, published data on the bioactivity of mushroom mycelia are limited compared with those of mushroom fruiting bodies.

In this chapter, we first discuss the antioxidant activity of 20 different species of wild mushroom mycelia [9]. These mushrooms are considered edible in the Tohoku area in northern Japan. Second, we present our findings on the ability of the comb tooth cap medicinal mushroom, *H. ramosum* mycelia, to promote NGF synthesis [9]. Finally, we discuss our results from using these mushroom mycelia in soybean fermentation [10] and discuss the prospects of utilizing *H. ramosum* mycelia in soybean fermentation for large-scale production of aglycons.
