Abstract: A cartridge valve assembly is slidably and sealingly receivable in a recess therefor in the housing of a pressure regulator for supplied air breathing apparatus. The cartridge valve is preassembled and slidably inserted as a unit in the pressure regulator housing recess, and the body is provided with an external shoulder engaged by a headed fastener in the housing to retain the cartridge valve assembly in the housing recess. The cartridge comprises a cylindrical body having a bore therethrough in which a valve stem is slidably supported for a valve element thereon to open and close the bore at one end thereof providing an air outlet for the valve. An air inlet port opens radially through the body into the bore, and the outer surface of the body carries O-ring seals on axially opposite sides of the air inlet port which sealingly engage the cartridge recess. The end of the body opposite the air inlet pivotally supports a valve stem actuating lever arm.