A filler material for a surgically implantable prosthesis comprised of a synthetic triglyceride having a viscosity substantially greater than that of naturally occurring triglycerides is provided. The triglyceride composition is formed of saturated alkyl chains to reduce or limit the potential for oxidation of the alkyl chains to form peroxide groups which adversely affect the biocompatibility of the filler material and the implant. Preferably, the filler material has a viscosity substantially the equivalent of a normal human breast. The synthetic triglyceride filler material may also have a lower viscosity which is a liquid at room temperature for use in inflatable devices. Prostheses containing filler material having the desired viscosity and antioxidation characteristics are also provided as well as a method for preparing the filler material.