The economics of the plutonium fueling of a thorium-cycle high temperature gas-cooled reactor (HTGR) have been investigated. This study showed that once-through cycles are more profitable than cycles with the 233U recycling in the fissile particles and cycles with the 233U recycling in the fertile particles, if limitation of age factors are applied and if core power densities are fixed. There is an economic advantage in using plutonium in once-through cycles once its price drops below 9 $/g Puf. The highest plutonium loading per particle provides the most attractive fuel cycle cost during the initial period when the fabrication costs are high. In the first irradiation test, which was carried out in the R 2 reactor (Studsvik-Sweden), burnups of 200 000 MWd/MTM 360 000 MWd/MTM at temperatures of 1850 and 1200°C were reached. In a second test, the center rods of two DRAGON reactor fuel elements were built with plutonium fuel. After irradiation equivalent to 224 days at full power, there was no damage to the particles. Finally, three batches of particles with diluted and undiluted kernels were irradiated during 45 equivalent full power days by KFA/Jülich. The postirradiation results were consistent with no fission gas release and no breakage event. A migration of plutonium occurred to a small extent up to the SiC layer in some of the particles.