Abstract: A method is described for priming an in-line molten metal filtration unit having a porous ceramic or refractory filter mounted substantially horizontally in a filter box. This filter box has an inlet for molten metal and an outlet for molten metal, the outlet being a closeable outlet in an exit well connecting to the downstream side of the filter. The method of the invention comprises the steps of adding a molten metal to the filter box sufficient to entirely cover the upstream side of the filter with a depth of molten metal and temporarily sealing the outlet in the exit well. A steadily increasing vacuum is then applied to the closed exit well at a rate between 0.1 and 10 kPa per second by withdrawing a stream of air from the exit well by means of a fan or air venturi until molten metal begins to flow through the filter. At that point the vacuum is quickly released and the molten metal outlet is opened.