A magnetic tape cartridge (11) is shown in which a length of magnetic tape (12) is wound on a reel hub (10) with the free end of the tape permanently connected to a generally rectangular-shaped leader block (13). The cartridge completely surrounds the reel of magnetic tape except for an opening (16) provided at one corner of the cartridge (11). This opening (16) permits tape (12) wound on the reel to be unwound as the leader block is moved away from the cartridge, preferably by an automatic threading mechanism. The cartridge comprises a leader block receiving well in which the leader block (13) is a snug fit. The walls (33, 51) of the well have the opening (16) in them through which the tape enters and leaves the cartridge. The walls (53, 33) of the leader block well are arranged to engage the leading and trailling walls of the leader block in a clamping relationship and help retain the block in the well in a position in which the opening (16) in the cartridge is substantially sealed by the leader block to prevent contamination from entering the cartridge when the cartridge is not in use. To remove the leader block from the well, it must initially be swivelled about boss (60) providing a mechanical advantage.