The hardness, Young's modulus of elasticity, tensile yield stress, ultimate stress and calcium content of 65 specimens from mammalian long bones and dental tissues were determined. The hardness was a very good predictor of the Young's modulus and yield stress, but a less good predictor of ultimate stress. The relationship in each case was nearly linear. All of these properties have the same type of non-linear relationship to the calcium content of the specimens. Over the range of calcium contents used here, hardness would be a useful guide to the mechanical properties of mineralized tissues in situations where conventional test specimens could not be produced, or where variations in mechanical properties over small distances are of interest.