John Caius (1510-75) enjoyed a European reputation as a Galenist physician. This study, based on his marginalia preserved in Eton and Cambridge, describes Caius' immense efforts to see and collate medical manuscripts in Italy and England over almost two decades. His reports are important for a modern editor of Galen, since many of these 'codices' are, apparently, now lost, and some were of high quality. Caius' notes also shed light on the growth of medical humanism, on the ac