The knee joints of mice can serve as a model for studying knee ligament properties. The goal of our study was to measure the structural stiffness of the medial collateral ligament (MCL) of the murine knee. A tensile test was developed for this purpose. First 84 femur-MCL-tibia complexes of 11-week-old C57Black6 mice were tested. Of four groups (n = 14 per group) the right MCL was ruptured. The mice were sacrificed at 1.5, 3, 6, and 9 weeks after the operation. The other two groups served as controls at 0 and 9 weeks after the operation. Absolute values of the structural stiffness of the healed MCLs at 1.5 weeks were initially significantly lower than their unoperated controls, but were not different from normal values at three, six, and nine weeks of healing. The structural stiffness of the unoperated controls increased by 11% at 20 weeks compared to 11 weeks of age.