A negotiable instrument such as a check includes a unique microprint identifier that allows for authentication while preventing unauthorized reproduction and alternation. A printing system generates the identifier after receiving a customer order for printing a plurality of negotiable instruments, to allow inclusion of information that is specific to the customer order and/or the printing process. In various embodiments, the identifier is unique to each or each subset of the plurality of negotiable instruments and facilitates authentication of each of the negotiable instruments when needed.