In this investigation, the effect of immobilization on the structural biomechanical properties of the immature rabbit medial collateral ligament (MCL) was quantified. The right hindlimbs often, three-month-old female New Zealand White rabbits were immobilized. The hindlimbs of the first group (n = 5) were immobilized for one month, until the rabbits were four months of age. The hindlimbs of the second group (n = 5) were immobilized for three months, until the rabbits were six months of age. Left hindlimbs were not surgically treated and served as contralateral controls. Fifteen normal animals were kept in similar cages until they were killed in subgroups at three, four, or six months of age. Biomechanical investigations after the animals were killed included the testing of a number of MCL structural properties, including prefailure and strength tests. Results showed that the maturation of MCL structural properties were inhibited during immobilization with signs of structural degradation becoming more obvious at the longer period of immobilization. Results also demonstrated that the structural mechanical properties of the contralateral MCLs were significantly altered. These results suggest that knee mobility is essential to the normal maturation of MCL mechanical properties and that contralateral MCLs cannot be used as normal controls in this immature rabbit model of immobility.