Uranyl ions (UO22+) stabilize black lipid membranes (BLM's) as inferred from the doubling of the breakdown voltage and from a considerable increase in the lifetime of the BLM's. These effects are observed also in BLM's made of mono-olein and of oxidized cholesterol. The lytic effect of lysolecithin is significantly reduced in the presence of UO22+. Uranyl ions adsorb to the interface of BLM's made of phosphatidylcholine (PC) with a dissociation constant of about 3 : 10-6 M and thereby charge the interface of the membrane and attain almost stoichiometric binding of one molecule of uranyl ion per one molecule of PC at 1 M ionic strength and 20 μM of UO22+. The membrane conductance induced by ionophores is considerably reduced by UO22+ and it is inferred by various tests that this is due to the charging of the interface and not to changes in membrane fluidity.