The invention relates to a method for testing the authenticity of a data carrier (1) and/or an external device (2) which enters into data exchange with the data carrier (1). According to the invention, the data carrier (1) and the external device (2) are each equipped with a special additional apparatus (4, 6) for generating and/or testing authenticity data. Data transmission between the data carrier (1) and the external device (2) as required for authenticity testing is performed at least partly via a special transmission channel (B). The transmission channel (B) for transmitting authenticity data is separated physically or logically from a transmission channel (A) for transmitting standard data so that there is no mutual interference of data transmission via the two transmission channels (A, B). In authenticity testing, the additional apparatuses for generating and/or testing authenticity data (4, 6) of the data carrier (1) and external device (2) and optionally also the transmission channel (B) for authenticity data must meet special demands which cannot be met by conventional designs. The transmission channel (B) for transmitting authenticity data is activable during the total time period between activation and deactivation of the data carrier (1) so that authenticity testing can be performed anytime.