This study presents morphometric analyses of collagen subfascicle area fraction and collagen fibril diameter distributions for the anterior cruciate (ACL) and medial collateral (MCL) knee ligaments from transmission electron micrographs of ligament cross sections of five mature, female New Zealand White rabbits. Statistically significant differences in subfascicular area fractions were found between the ACL and MCL (0.89 +/- 0.02, 0.97 +/- 0.01, respectively; p less than 0.001). Mean fibril diameters for the ACL and MCL were also significantly different (0.059 +/- 0.005, 0.085 +/- 0.011 microns, respectively; p less than 0.025). Fibril eccentricity (a measure of parallel alignment of collagen fibrils within the ligaments, defined as the ratio of minor to major axes of elliptical fibril outlines) was 0.89 +/- 0.03 and 0.85 +/- 0.08, respectively, for the ACL and MCL; these data were not significantly different (p greater than 0.1). The relative amount of variation in the pooled fibril diameter data due to variation between animals, ligaments, locations within ligaments, and among fibrils at individual locations are reported. The variation of fibril diameter distributions between the ACL and MCL was substantially greater than the variation between different locations within each ligament cross section as well as between different animals. The structural differences reported may help explain known differences in the biomechanical properties of the ACL and MCL.