Two Josephson voltage standard (JVS) systems operated at the National Institute of Standards and Technology (NIST) and Lockheed Martin Astronautics (LMA) were compared by using four traveling Zener standards. A Measurement Assurance Program (MAP) protocol was adopted for the comparison. The Zener data were first corrected based on their pressure coefficients to compensate for the pressure difference due to the lab elevations and local meteorological conditions. The Welch-Satterthwaite formula and effective degrees of freedom (DOF) were then used to calculate the expanded uncertainty. The mean difference between the measurements of the two laboratories was found to be 0.059 /spl mu/V with an expanded uncertainty of /spl plusmn/0.189 /spl mu/V at the 95% confidence level.