The relationship between the mechanical properties of bone in three-point bending and eight histocompositional variables was studied. Ultimate stress, ultimate strain, and elastic modulus were measured in 35 beams of cortical bone from bovine tibias using standard ASTM methods. Four elements of porosity were determined by point counting, mineralization by ashing, and wet and dry apparent density from weight and volume. Collagen fiber orientation was estimated using polarized light, and specimens were categorized as plexiform, mixed, or osteonal. Analysis of variance showed that ultimate stress was similar in the plexiform and osteonal specimens, but elastic modulus was reduced in the latter (18.6±1.2 vs 21.0±1.9 GPa), which were significantly less porous (by 24%) and less mineralized (by 3%) than the plexiform group. Stepwise multiple regression analysis showed that collagen fiber orientation ranked highly as a predictor of bending properties. The next best predictors were density and mineralization. In the plexiform group, 77% of the variability in elastic modulus was accounted for by wet and dry density and collagen fiber orientation. In the osteonal group, 88% of modulus variability was accounted for by percentage mineralization and collagen fiber orientation. When all the specimens were pooled, 62% of the variability in elastic modulus was attributable to variations in collagen fiber orientation, density, and porosity due to Haversian canals.