Apparatus for circular surgical stapling of hollow organs comprising an instrument and disposable cartridge assembly. The instrument comprises a housing with a throughbore in which a tube is received for reciprocation by means of a manually operable squeeze handle. A rod reciprocates within the tube by means of a wing nut cooperating with a threaded part of the rod. A two-part disposable cartridge assembly comprised of an anvil-carrying part and a staple-carrying part are detachably mounted on the rod and housing, respectively. Keys are provided to hold the anvil-carrying part and the staple-carrying part in rotational orientation relative to the rod. Mutually coacting stops on the tube and rod determine the extent of reciprocation which is variable in dependence upon the position of the rod which, in turn, is dependent upon the juxtaposition of the anvil-carrying part with the staple-carrying part when the apparatus is closed on tissue preparatory to effecting a circular anastomoses. A novel geometric configuration for the anvil-carrying part is provided for easy removal of the instrument from a stapled anastomosis. Further, the interior of the stapling mechanism is generously vented to prevent the build up of excessive pressure in tissue confined within the stapling mechanism.