Dr Letham is distinguished for his discovery, naming, and characterisation of the cytokinins, a class of hormones that control cell division and other aspects of development in plants. Of the 20 identified cytokinins now known from higher plants, 15 were isolated, characterised chemically, and unambiguously synthesised by Dr Letham and his colleagues. Cytokinins are now known to be a major class of plant hormones, and their study has become a large branch of plant science, prosecuted world-wide, and stemming from Dr Letham's seminal work. He has used sophisticated techniques of mass spectrometry to elucidate their structure and pathways of metabolism. He and co-workers have also provided the first unequivocal identification of cyclic adenosine monophosphate in higher plants - another significant landmark. He is co-editor of a book on Ribonucleic Acids and a two-volume treatise on Phytohormones, both widely acclaimed.