Abstract: A method is described for testing the transmission quality of a data transmission system in its operational state. During a test key actuation data bits having a specific marking polarity and at least one additional bit having a polarity corresponding to the spacing condition are transmitted. At a receiving station the latter transmitted bit group is analyzed for disturbances. In the event of multiple undisturbed reception of the aforementioned bit pattern a visual indication is produced. At the same time, in the receiving station, the data output of the receiver is connected to the transmitter, and the aforementioned bit pattern is transmitted back to the testing station. With multiple undisturbed reception of the bit pattern a visual indication is produced in the testing station. During the test procedure the data input and output units are blocked.