The effect of isotope exchange on the Néel temperature in the cuprates was neer determined because it was impossible to perform relaible isotope exchanges in the parent compound. This problem might have been solved recently with the characterization of the (Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy (CLBLCO) phase diagram. There are two regions in which the critical temperatures are independent of oxygen content: the first is the usual optimal doping; the second is at extreme underdoping. In this experiment we would like to check the isotope dependence of TN in CLBLCO for the parent compound and the penetration depth for optimally doped compounds. This experiment could play an important role determining the origin of high temperature superconductivity.