Specimens of bovine femur were loaded in tension at strain rates of from 0.00013 to 0·16 sec−1. The ash content and amount of reconstruction (Haversian bone) of each specimen was determined. Yield stress, ultimate tensile stress, modulus of elasticity and strain at yield were all greater at greater strain rates and at greater mineralization. An opposite effect on all these variables was produced by reconstruction in the specimen. These effects were nearly all ‘important’ in that the effect of varying the independent variables over the experimental range altered the dependent variables by usually over 30 per cent. Strain rate on one hand, and ash content or reconstruction on the other, account for a considerable proportion of the total variance in the mechanical properties. Statistical analysis of the results suggests that the effect of reconstruction on mechanical properties is caused by its reduction of the mineral content of the bone, and not by other effects it may have on the structure of bone.