Methods are described for the detection and estimation of guanidinoxy compounds, including canavanine, in biological material. Canavanine has been isolated from the seeds of the bladder senna, Colutea arborescens. A guanidinoxy compound, similar in its colour reactions to canavanine, has been detected in the seeds of Medicago arborea, M. echinus, and Ornithopus perpusillus.