An apparatus for automatically marking laboratory test tubes in the stage of drawing biological material to be analyzed, which includes two or more distributors of test tubes of different types that are selectable by a control unit and are suitable for feeding test tubes to a test-tubes positioning device which in turn is suitable for presenting correctly positioned test tubes to a computerized labelling machine containing a printer for printing barcodes on labels. Each distributor includes an automatic drawing and discharging device for drawing and discharging single test tubes from a container of test tubes of a selected distributor to the positioning device.