This method describes a procedure for the sizing and counting of particulate contamination in liquid samples by membrane filtration. The procedure will allow measurement of particulate contamination five micrometres or greater in size with a maximum variation of ±20% in results over an average of two runs. This procedure can be used for all samples where the membrane filter is compatible with the sample liquid and rinse liquid. Section II of this procedure may be used to count any sample on a gridded membrane where particles are evenly distributed. This procedure is an alternative to counting with an automatic particle counter although results by each method from identical samples might not be equivalent due to individual idiosyncrasies in each technique.