Rotating anode x‐ray tubes for medical roentgenography require bearing operation in high vacuum. A method of lubricating such bearings by vaporizing a thin film of low vapor pressure material, such as pure barium metal, onto them in the final vacuum is described. The results of tests with a great many different metals as the vaporizing films and combination films, as well as different base materials, are given. Experimental evidence is presented that indicates the lubricating film to be a liquid alloy of barium and metals such as cobalt, chromium, or aluminum.