On 7th November 1545, Calvin wrote a letter to a(n unnamed) friend, which has been included in the publication of all his works by the title “De usuris” (About the interestrate). The current study analyses this letter as the most comprehensive summary of Calvin’s judgement on interest rate. It is a well-known fact that based on statements in the Old Testament and the New Testament (Deut. 23/19, Luke 6/35), he not only banned thecollection of interest, but he did also not differentiate between interest and extortionate. The prohibitions of the Early Middle Ages were confirmed by the scholastic Fathers of Church as well as the legislation of the Synod, referring to Aristotle. (Corpus iuriscanonici, II. Lateran synod, 1139. The principle of “pecunia pecuniam parare non potest” was, however, infringed between the 13th and 16th Centuries in a number of ways. Basically, the collection of interest was disguised as ship loans (nauticum foenus), bills (cambium), and reverse bills (ricorsa), orbusiness deposit (depositum). All these, however, did not change the standpoint of the Church, even though they were widely used. Pope Gregory IX declared shipping loan extortionate already in the 13th Century; and deposit was condemned by V. Pius even in 1571. Most of the 16th Century Reformers shared similar views and did not approve collecting interest. Luther stuck to the principles of the Canon Law and condemned those that had not paid their debt referring to the prohibition of usury, while his opponent, Johannes Eck, argued for the collection of interest. The above-mentioned letter of Calvin meant a decisive turning point, and it made unnecessary for his followers to disguise the collection of interest in the usual formats. Following the ’sola scriptura’ principle, he denied the validity of the statements of the Fathers of Church claiming that money did not bear money. The sea or the earth did, it was only money that did not? And analysing the words of the Holy Bible (examining not only Moses and Luke, but the psalms and prophets, too), he concluded that “there was no evidence in the scripture regarding the fact that all interests would be censurable”. In this respect, it is not advisable to tie our conscience more than God wants us to. He still doesnot support if someone pursues lucre with interest as a job. He mentions seven exceptions that have to be kept when collecting interest, including the justness of the lender or that one should not ask for more interest than allowed by “the laws of the country or the location”. All this refers to the fact that he separates lawful interest from extortionate for the first time, and he also differentiates loan for the purpose of consumption from loan supporting the purpose of enterprise – he only does not approve the first case.