The catheter insertion valve of the present invention comprises a central tubular body and a non-removable tubular cap that can be screwed onto the body wherein a bore extending through the body and the cap defines a pathway for a catheter. Such catheters are inserted into arteries, organs or body cavities for the purpose of introducing medications of instruments for treatment, diagnosis or the performing of in vivo procedures. A seal located within the central body of the valve is used to form a fluid-tight seal about the catheter, guidewire or other instrument extending therethrough. Alternatively, the seal can completely close off the pathway when it is not being used for catheter insertion thereby preventing air entrainment. The central body is provided with one or more seal position indicators to indicate to the operator that a seal has been formed about a catheter or guidewire of standard dimension. The seal is manually adjusted by rotating the non-removable cap. The cap is formed on a mold, removed prior to cooling and then reformed by annealing the cap prior to mounting onto the central body of the valve.