An unusual amino acid, L-methionine sulfoximine (1), has been isolated from the fresh seeds of Cnestis palala (Lour.) Merr. [Connaraceae]. The absolute configuration of the natural sulfoximine (1) was confirmed to be 2(S)-methionine S(S)-sulfoximine [(2S,SS)-2-amino-4-(S-methylsulfonimidoyl)-n-butanoic acid] by comparison of the [alpha]D value and IR spectrum with those of authentic samples obtained through the optical resolution of synthetic materials. Acute toxicity of the seeds of C. palala in a beagle dog was also studied.