Abstract: A device capable of sealing the extracorporeal end of a catheter sheath in which has been disposed a catheter is described. An elastomeric member such as a toroidal bladder is confined within a chamber defined by a rigid, hollow body portion and controllably distended so as to restrict a lumen through which the catheter passes, thus effecting a seal against the egress of blood in course of catheterization. Means are provided to permit the admission of fluids or withdrawal of blood samples, etc. at the proximal end of the catheter sheath simultaneously with catheterization.