Abstract: The invention relates to a method for the secured use of transportable data storage media in a closed network. The invention further relates to a transportable data storage medium comprising a rewritable memory for use in a closed network and to a closed network. The closed network (1) formed of at least two data processing devices (2, 3) is contactable by a transportable data storage medium (4). In order to allow data exchange between the closed network (1) and the transportable data storage medium (4) only when the transportable data storage medium (4) is also authorised for use, respective programs (7, 8, 9) are installed on both the data processing devices (2, 3) and the transportable data storage medium (4), by means of which programs an identification data set (7.1) stored on the transportable data storage medium (4) can be found, checked, transmitted, compared with a comparison data set (9.1) and can be replaced by a new identification data set (7.2).