NOVELTY - The average size of metallic compound particles or agglomerates suspended in a fluid is reduced by flowing the fluid one or more times through magnetic field(s) to reduce the average size of a portion of the metallic compound particles or agglomerates by at least 25%, preferably at least 50%.    USE - Reduction of the average size of metallic compound particles or agglomerates suspended in a fluid, e.g. organic solvent or water. The ultrafine particles are used in catalysts (e.g. for treatment of incineration gas or gasoline combustion gas in vehicle exhaust), magnetic recording media, optoelectronic materials (e.g. X-ray photoresists), magnetic fluids, composite materials, as coloring agent in concrete, thermoplastic and thermosetting (co)polymers, powder paints, rubbers, elastomers and bitumens, in semiconductors (e.g. forming a metallic deposit in precise pattern on titanium dioxide), rechargeable batteries, fuel cells, ceramics, toner compositions, bone cement and dental restorative.    ADVANTAGE - The process prevents agglomeration, broad particle size distribution and low volume production.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for increasing the intensity of the color of a material including metallic pigment particles or agglomerates, comprising incorporating particles or agglomerates to the material, where the average size of a portion of the particles or agglomerates is 0.5 nm to 5 mum;    (2) the material including the metallic pigment particles;    (3) an industrial process using the metallic compounds;    (4) a method for evaluating the performance of a magnetic treatment comprising flowing fluid containing metallic particles or agglomerates through magnetic field(s) while:    (a) controlling the size of the particles or agglomerates or the turbidity of the suspension;    (b) flowing through the magnetic field(s); and    (c) controlling the size or turbidity after magnetic treatment; and    (5) apparatus for affecting the particle size reduction.    DESCRIPTION OF DRAWING(S) - The figure shows a reservoir system for carrying out the magnetic treatment.    Tubing (1a-1c)    Internal magnet (2)    Pump (4)