A two-fluid theory of superconductivity that was derived from an analysis of covalent interactions in the polaronic high-T-c cuprates is used to predict the penetration depth of YBa2Cu3O7 as a function of temperature. In a related study, the model is also applied to the temperature dependence of microwave surface resistance measurements on both YBa2Cu3O7 and Nb films. These results suggest basic differences between oxide and metal superconductors.