Twenty-eight male and twenty-one female subjects with no history of previous injury to their knees were examined using a newly developed clinical testing apparatus designed to record anterior-posterior tibial force versus displacement and varus-valgus moment versus angulation during manual manipulation of the knee. Joint stiffness and laxity were measured from test tracings made with the knee muscles relaxed and tensed. Agreement between these measurements and those made previously on thirty-five fresh cadaver knee specimens was very good. Anterior-posterior laxity averaged 3.7 millimeters in full extension, 5.5 in 20 degrees of flexion, and 4.8 millimeters in 90 degrees of flexion, while the mean varus-valgus laxity was 6.7 degrees in full extension. The common clinical assumption that normal right-left differences are negligible was found to be invalid. Individual right-left differences averaged 26 to 35 per cent for laxity and 19 to 24 per cent for stiffness. There was no discernible tendency for one knee to be more stable than the other; random interchanges of relative stability between the right and left knees were observed for each individual at different knee positions. When requested to tense the knee muscles, these subjects were able to increase their knee stiffness an average of two to four times while knee laxity was reduced to 25 to 50 per cent of the normal value.