A method and apparatus for mixing bone cement (96) which produces bone cement (96) essentially free of entrained air. The apparatus includes a cartridge (30) having two telescoping cylinders (44,46). The outer cylinder (44) receives a cap (32) thereon which hermetically seals the cartridge (30). The cap (32) includes an outlet port (62) in which a break-away agitator (34) is installed and a port (60) for connection of a vacuum pump (24). In use, a desired vacuum pressure is drawn on the cartridge (30) and the inner cylinder (46) is allowed to be drawn into the outer cylinder (44) while the vacuum is being drawn, whereupon the inner cylinder (46) automatically stops moving relative to the outer cylinder (44) when the desired vacuum pressure is reached. Mixing of the bone cement (96) is then performed under the partial vacuum thereby created. The agitator paddles (74) provide a scraping action against both inner cylinder walls of the telescoping cylinders (44,46) thereby improving mixing. After mixing, the agitator (34) is broken away, an appropriate nozzle (36) is attached to the cartridge (30) and the cartridge (30) is inserted into a cement gun. The leading portion of the bone cement (96) develops a convex profile when axially advanced through the cartridge (30) and the piston plug (48) includes a tapered protrusion (92) which generally corresponds to the profile of the bone cement (96). The cap (32) and paddles (74) are also shaped correspondingly to the convex profile developed by the bone cement (96), resulting in substantially all of the bone cement (96) being squeezed from the cartridge (30). An air escape passage (66) is also provided to vent air contained within an annular gap between the agitator shaft (70) and the cap (32) upon initial expulsion of bone cement (96), resulting in the bone cement (96) expelled from the cartridge (30) being essentially free from entrained air.