A cartridge for injecting a mixture of two liquid constituents comprising a reservoir (1) which has a compartment (2) for one of the constituents and a compartment (3) for the other constituent. The reservoir (1) comprises two concentric cylindrical compartments (2, 3), respectively containing a first and a second constituent. A piston (6, 7) is mounted in sliding manner in each of the two compartments (2, 3), and provision is made for sliding the two pistons (6, 7) simultaneously. Each of the compartments (2, 3) opens into a common mixing chamber (4) which comprises partitions (8, 9) defining a sinuous path of the mixture of the two constituents between the two compartments (2, 3) and the outlet opening (5) of the mixing chamber (4). Use, in particular, for injecting a semi-pasty sealing material into a hollow body.