A housing retains a lancet body which encloses a needle whose tip is covered by a cap. The cap has a head provided with flanges. These flanges locate within notches at the end of the housing on only two sides of the housing. The location of the flanges within the notches holds the lancet within the body, before use, so as to compress a spring positioned between a head and a slotted portion of the housing. When the device is to be used, the head of the cap is rotated through 90° so as to detach the cap from the rest of the lancet body and release the flanges from the notches. This allows the lancet body to be actuated by a sprung-loaded trigger-release mechanism, when required, so that the tip of the needle projects momentarily through the opening at the end of the housing and then bounces back.