olydisperse glass microsphere standards were analysed by 25 international teams of particle metrology specialists using unambiguous methods such as microscopy, sedimentation, electroformed sieving and Coulter counting. The nominal size distributions were: 1-10, 3-30, 10-100 and 50-350 microns. The Laser Diffraction Proficiency Testing Scheme (LDPTS) then sent the samples to about 65 participant laboratories for analysis using a wide range of laser diffraction particle sizing instruments. The standards were spin riffled to minimise sampling error. Excellent accuracy and repeatability was achieved in the homogeneity testing by Beckman, Coulter and Malvern laser diffractometers. Although some laboratories produced seriously flawed data, when the average sizes were calculated corresponding to the values of the sizes corresponding to the 10th percentile (D10), median (D50) and 90th percentile (D90) of the cumulative volume-weighted size distributions, there was good agreement with the primary particle size data.