Emission of noxious nitrogen oxides with the flue gas from the regenerator of a fluid catalytic cracking plant are reduced by incorporating into the circulating inventory of cracking catalyst separate additive particles that contain a copper-loaded zeolite. A particularly effective additive is provided, for example, by treating a ZSM-5 zeolite in a matrix consisting predominantly of titania, zirconia or a mixture thereof with a solution containing both copper and rare earth cations to incorporate both of these elements with the zeolitic composition.