This paper is concerned with a novel class of oxygenators which Li and Asher first proposed and called "liquid membrane oxygenators." They are different from usual bubble blood oxygenators in that oxygen bubbles are individually encapsulated by a liquid fluorocarbon membrane and dispersed in blood, instead of bare oxygen bubbles normally used. The fluorocarbon membrane's role is to prevent direct contact of blood with oxygen gas which can cause a rupture of red blood cells and denaturation of proteins in blood. We have attempted to produce encapsulated bubbles by use of a fine double-tube nozzle where oxygen is supplied through the inner tube of the nozzle while a liquid fluorocarbon is supplied through the annular space between the outer and the inner tubes. This device has been found to enable steady formation, both in pure water and in an aqueous solution of bovine serum albumin, of encapsulated bubbles of about 2 mm in diameter, which are far smaller than those produced by Li and Asher's original device. A comparative study of oxygenation of water (a substitute for blood) by encapsulated bubbles and by nonencapsulated bubbles indicates that the former is only slightly inferior to the latter.