The toxic principle of Cnestis glabra, identified as methionine sulfoximine has been found as a permanent constituent of the soluble amino acid pool of this plant. In the roots the toxin concentration varies markedly according to the developmental stage of the plant.The poisonous principles of Cnestis polyphylla and Rourea orientalis, two other Connaraceae species, were purified to homogeneity. Similarity of (1) their physiochemical and biological properties, (2) their behaviour in thin-layer and gas chromatographies and (3) their infrared spectra to those of the toxin of Cnestis glabra established their identity to methionine sulfoximine. This methionine derivative, never reported as a natural product, seems to be characteristic of toxic Connaraceae species.