The Cely Letters is a well-known collection of correspondence exchanged by members of this London family of wool merchants and their associates between 1472 and 1488. A substantial part of the corpus was written and received by factors based in Calais, which had been an English outpost in France since 1346 and was strategically connected to the wool marts of the Low Countries. The great majority of the letters are monolingual English texts, thus attesting to the widespread use of the vernacular in personal correspondence by the late fifteenth century. Nevertheless, behind the monolingual English surface, traces of multilingualism are revealed. In this paper, I intend to analyse this issue with a twofold purpose. In the first place, attention will be paid to the multilingual background of the letters, considering both the persistent use of French in late medieval England and the specificity of the business transactions carried out at Calais and the marts, where language contact must have been the norm. In the second place, different textual reflections of such contact in the letters are examined and classified, both as regards the generic conventions of letter writing and as part of the multilingual business context where they were produced and received.