A respiratory suction catheter assembly includes a manifold supporting a rotatable swivel member for engagement with an endotracheal tube, and a seal support body disposed on a boss of the manifold opposite the part supporting the swivel for supporting a catheter tube seal and a porous filter for filtering air flowing into and out of a flexible elongated sheath enclosing the catheter tube. The sheath includes opposed tapered ends secured to the seal support body and to a boss on a suction control valve housing. The valve housing supports a tapered plug closure member engageable with a resilient actuator and a slide-lock member. Certain components, including the sheath, the seal support body, the filter, the catheter tube seal and the catheter tube may be treated with an anti-microbial agent.