A security transfer unit includes a hollow box-structure (1) that has open ends (10 and 11) on opposite sides of a security barrier (2) and is supported from side-frames (4 and 5) with a clearance underneath. A rolling shutter (12) encircles the box-structure (1) to block access at either end (10 and 11) to the chamber (3) through openings (7 and 9) in the unit casing (6). The shutter (12) is driven to bring an aperture (19) therein into register with the openings (7 and 9) in turn so that access to the chamber (3) can be gained through only one of them at a time. Drive is in one direction only and is interrupted with the shutter (12) blocking both openings (7 and 9) when the aperture (19) is located in register with an aperture (22) in the box-structure (1), so as to enable viewing of the contents of the transfer chamber (3) through a window (23) above.