A cassette (10a, 10b, 11), which contains juxtaposed wire staple blanks releasably interconnected to form at least one strip (8) of staple blanks, is adapted to be used in a stapler for driving staples into an object, such as a sheaf of papers. The stapler comprises a base and a stapler head movable in relation to the base. The cassette accommodates a reciprocating staple shaper (20), adapted to successively shape the staple blanks of the strip (8) into U-shaped staples, and a reciprocating staple driver (19), adapted to successively release the thus-formed staples from the strip (8) and drive them into an object. The cassette has a front wall (11) in which the staple shaper (20) and the staple driver (19) are displaceably guided, as well as recesses through which projections on the staple shaper and the staple driver project, so as to engage, when the cassette is mounted in the stapler head, a drive mechanism (4) arranged in the stapler head to reciprocate the staple shaper and the staple driver.