a-(Hydroxymethyl)serine was isolated and characterized from the seeds of Vicin pseudo-orobus (Leguminosae). Glabrin, the toxic principle of Cnestis glabra isolated from root bark, was identified as S-(3-amino-3 carboxypropyl)-S-methyl sulphoximine (methionine sulphoximine) by spectroscopic and chemical means. The natural occurrence of this toxic methionine derivative is reported for the first time.