The effects of the well established carcinogenic metals or metalloids (As, Be, Cr, Ni), hypothetically carcinogenic but well-established cocarcinogenic metals (Cd, Pb, Co) and weak co-carcinogenic metals (Al, Cu, Fe, Zn) and the antagonism between Mg and these metals were studied on the ionic transfer through the isolated human amnion. The ionic transfer is estimated by the measure of the membrane potential (delta psi), the cationic number transference t+ and the ionic fluxes (F1, F2). All carcinogenic metals increase delta psi (+) and reduce t+, F1 and F2. Mg has an opposite action with regard to Cd, Pb and Ni, but may act as a co-carcinogenic agent by increasing the effects of Al, Cu and Zn.