Abstract: A method of preparing milled solid particles of a compound comprises the steps of forming a slurry of a liquid medium, the compound and rigid milling media in a milling chamber, contacting the compound with the milling media while in the chamber to reduce the particle size of the compound, and thereafter separating the compound from the milling media by vacuum filtration through a removable filter probe immersed in the slurry. In a preferred embodiment, the milling media is a polymeric resin having a mean particle size of less than 300 .mu.m. The method enables the use of fine milling media which provides extremely fine particles of the compound while avoiding problems, e.g., decrease yields due to separator screen plugging, associated with prior art processes.