Abstract: A flexible fiber optic probe or catheter insertable into the cardiovasular system for monitoring blood oxygen saturation or the like is provided with an improved standard for the calibration of the catheter and its associated electro-optic equipment. The improved calibration standard is particularly suited for catheters of very small diameter, is provided by a generally tubular reflecting member aligned with and adjacent to the distal end of the catheter. The reflecting member may be vinyl tubing or the like which may be removably of fixedly positioned about the distal end of the catheter to reflect light directed thereon from the catheter when in air or a clear sterile solution for calibration.