The strength and failure mode of a fresh femur-anterior cruciate ligament-tibia complex was studied in normal rabbits and after the induction of an experimental immune synovitis. Five modes of failure were observed: (1) through the anterior cruciate ligament alone; (2) through a small piece of bone with the anterior cruciate ligament attached; (3) through a condyle; (4) through the metaphysis; and (5) through the diaphysis. In the normal complex, failure always occurred through the anterior cruciate ligament. In the controls, the average ratio of failure-load to body weight was seventy-four newtons per kilogram. After induction of an immune synovitis this ratio was significantly reduced, to twenty-two newtons per kilogram. Synovial cathepsin-D enzyme activity in these knees was 5.27 micromoles per hour per milligram of protein, which was significantly elevated when compared with the control level of 0.26. Histological examination of the anterior cruciate ligament after induction of the immune synovitis but before strength-testing demonstrated loss of the normal undulating fiber orientation, disorganization of the normal cellular pattern, and a decrease in the staining of the interstitial matrix with Mallory trichrome. There was a moderate infiltration of mononuclear and polymorphonuclear inflammatory cells in the body of the ligament. Histological studies of the ligament after strength-testing showed that the failure resulted in a mop-like appearance and morphological characteristics similar to those observed before testing.