An automatic apparatus is described, which is suitable for the withdrawal of portions of biological material from a parent test tube (1), mobile in a conveyor (6) of a test tube transport plant by means of transport devices (8) of single test tubes, to be loaded in one or more child test tubes (2) countermarked beforehand with suitable identification means and to be sent to different biological material analysis instruments interfaced to said transport plant. Said apparatus comprises a work bench (5) which provides a first lane (9) for stocking and filling of transport devices (8) with empty child test tubes (2), a second lane (10) for stocking parent test tubes (1), said lanes (9, 10) being selectively joined in such a way to consent the sending of single parent test tubes (1) with a predefined number of child test tubes (2) subsequently assigned to be filled with biological material drawn from the parent test tube (1) in a work point (11), and a return third lane (12) of parent test tubes (1) and child test tubes (2) filled with portions of biological material drawn from parent test tubes (1), said lanes (9, 10, 12) joining to a conveyor (6) suitable for the transport of test tubes to and from subsequent processing modules, a recruitment device (19) of empty child test tubes (2), marking and loading devices (3) of empty child test tubes (2) into empty transport devices (8) stocked in said first lane (9), a device (4) suitable for the withdrawal and distribution of portions of biological material from the parent test tube (1) to the respective child test tubes (2) queued to the idle parent test tube (1) at the work point (11) of the work bench (5), a recruitment and loading device (22) of pipettes (23) suitable for the withdrawal of portions of biological materials and their distribution in child test tubes (2), and a control unit (100) suitable to coordinate and check the devices involved in the work cycle of the described device.