A new amino acid, L-2-amino-7-hydroxyoctanoic acid was isolated from fruiting bodies of Russula cyanoxantha. Automated amino acid analysis [1] revealed the occurrence of an unusual amino acid in fruiting bodies of the mushroom Russula cyanoxantha (Schw.) Fr. Compound 1 was isolated from 70% EtOH extracts of the edible mushroom by ion-exchange chromatography and Avicel cellulose chromatography. From elemental analysis and SIMS (m/z 176 [M + 1]⁺), the molecular formula of 1 was estimated to be C₈H₁₇NO₃. The copper complex test [2] indicated that 1 is an α-amino acid. ¹H NMR in D₂O showed a three proton doublet at δ 1.17 which was attributed to the terminal methyl group. The multiplet between δ 1.2 and 1.7 (6H) accounted for three methylene groups (H-4, H-5 and H-6) and a multiplet between δ 1.7 and 2.1 (2H) was attributed to a β methylene group. The NMR also showed a triplet at δ 3.85 due to a α methyne proton and a multiplet between δ 3.6 and 4.1 which was attributed to a methyne proton strongly deshielded by an attached hydroxyl group. In addition, the ¹³C NMR showed eight carbon signals attributable to one methyl, four methylene, two methyne and one carboxyl groups. These results suggested that the most likely structure of 1 was 2-amino-7-hydroxyoctanoic acid. Oxidation of 1 with acidic potassium permanganate formed an amino acid 2 with Mᵣ 173 (SIMS m/z 174 [M + 1]⁺) which corresponded to the elimination of two protons from 1. ¹H NMR of 2 showed a multiplet between δ 1.2 and 1.7 (4H) due to two methylene groups (H-4 and H-5) and a two proton multiplet between δ 1.7 and 2.1 attributed to a β methylene group. A three proton singlet at δ 2.23 and triplet at δ 2.63 (2H) both deshielded by an attached carbonyl group were assigned to the terminal methyl and an H-6 methylene, respectively. A triplet at δ 3.87 (1H) was also attributed to α methyne proton. The strong absorption band at 1710 cm⁻¹ in IR spectrum of 2 showed the presence of a carbonyl group in the molecule. Thus, it was possible to identify 2 as 2-amino-7-oxooctanoic acid. This fact strongly supports the above structure of 1. The specific rotation of 1 was more positive in acid solution than in water, indicating that the amino acid belonged to the L-series at the α asymmetric centre. The configuration of the hydroxyl group in 1 is still unknown; β-Hydroxy-L-valine was isolated from fruiting bodies of Pleurocyhella porrigens (Fr.) Sing. In a previous paper [1], we reported the free amino acid contents of fruiting bodies of 113 mushroom species, as determined by automatic amino acid analysis of their 70% ethanol extracts. In the course of the experiment, some of the mushrooms were found to contain several unusual amino acids. This paper describes the isolation and identification of an unknown ninhydrin positive compound, hereafter termed as 1, from the fruiting bodies of the edible mushroom, Pleurocyhella porrigens (Fr.) Sing. Compound 1 was detected on the chromatogram of the amino acid analyser (Li-citrate buffer system) and was completely overlapped with threonine. The two were distinguishable by TLC (silica gel and Avicel cellulose). The isolation of 1 was accomplished by chromatography using several ion-exchange resins, Avicel cellulose and silica gel, followed by crystallization. From the elemental analysis and SIMS, the molecular formula of 1 was estimated to be C₅H₁₁NO₃. The ninhydrin reaction of 1 on paper was completely inhibited by Cu²⁺, showing that 1 is a α-monoamino acid. On the ¹H NMR of 1 (in D₂O), one gem-dimethyl group (δ 1.26 and 1.47) and one α methyne singlet (δ 3.65), which indicated the β carbon atom to be fully substituted, were observed. The absorption band at 1167 cm⁻¹ in the IR showed the presence of a tertiary hydroxyl group in the molecule [2]. These facts strongly suggested that 1 is β-hydroxyvaline. This was further supported by the formation of valine from the reduction with HI-red P [3], and the detection of glycine in the degradation product by barium hydroxide [4]. Optical rotation measurements on 1 performed in H₂O and 6 M HCl solution showed the shift to more positive rotations in acid associated with an L configuration. The IR of 1 correlated well with that of a synthetic specimen of β-hydroxy-L-valine [5]. Although β-hydroxy-L-valine was first found as a constituent of the antibiotic YA-56 [6, 7], it is thought that the evidence presented here is the first to demonstrate its natural occurrence in a free form.