An apparatus for automatically closing test tubes containing biological material mounted to an automatic test tube conveyor is described, comprising a cap recruiting device (11) adapted to feed a curved channelizer (12) adapted to tip a cap (5) from a configuration with the opening upwards at the inlet of the channelizer (12) to a configuration with the opening downwards at the outlet of the channelizer (12) at a loading surface (13). A horizontal pusher (22) moves the cap (5) from said loading surface (13) to a housing (23) in a channel (14) in which a stem (24) vertically slides and is adapted to move the cap (5) upwards in a test tube closing device (9) comprising a movable head (26) adapted to grip the cap (5) pushed by the stem (24) and to close the open test tube blocked at a working point of the conveyor.