Disclosed is an assembly for mixing fluids (i.e., gases or liquids), and more particularly an assembly that accurately mixes two or more high-pressure fluid sources and is adapted for use in applications, such as for example, chromatography. The mixer assembly (100) includes, inter alia, a housing (10), an inlet fitting (40), and a mixer cartridge assembly (60). The housing (10) has a fluid receiving section (16) and a fluid discharge section (18) with an outlet (20) formed therein. A central bore (22) extends between the fluid receiving section (16) and fluid discharge section (18). An inlet fitting (40) is engaged with the housing (10) and has first (42) and second (44) fluid ports formed therein that extend from the fitting exterior to the fluid receiving section (16) of the housing (10). A mixer cartridge assembly (60) is disposed within the central bore (22) of the housing (10) and is positioned between the inlet fitting (40) and the downstream end portion of the housing (10). The mixer cartridge assembly (60) includes a body portion (64), a plurality of spheres disposed within a central mixing chamber (62) formed in the body portion (64), and mechanism for retaining the spheres in the mixing chamber (62).