Experimental determination of the strain distribution in prismatic, square cross-section bars of human compact bone in torsion disclosed nonclassical effects associated with the microstructure. Specifically, in wet bone at small strain, significant deviations from the classically predicted strain distribution were observed. The measured strain distribution in wet bone followed predictions based on Cosserat (micropolar) elasticity. In dry bone, the strain distribution was very close to the prediction of classical elasticity. The interaction between Haversian osteons and the cement substance between them was hypothesized to be the principal mechanism for the phenomena. To evaluate this hypothesis, additional specimens were subjected to prolonged torsional load and the cement lines were observed by reflected light microscopy. Localized deformation at the cement lines was observed, but it was less than values reported earlier for bovine plexiform bone.