This study provided standard reference materials for fiber-counting by phase-contrast microscopy (PCM). PCM is subject to many sources of variation, including those dependent on the microscopist, so reference standards cannot be produced that are traceable to national or international standard units. Consensus standards using a “true value” agreed on by a number of laboratories may be acceptable. Reference slides for fiber-counting can be prepared using a proprietary process of grid overlay in which the fields of view defined by the grids are identifiable and relocatable. Multiple microscopists then can examine exactly the same areas of samples, reducing one source of potential variation. Twelve slides prepared from proficiency test samples of the American Industrial Hygiene Association (AIHA) Industrial Hygiene Laboratory Quality Program were used in this study, four each of chrysotile asbestos, amosite asbestos, and man-made mineral fibers. Five microscopists from AIHA-accredited laboratories, plus the inventor of the grid process, examined the slides in a blind study. This group represented commercial analytical companies, in-house corporate laboratories, research institutions, and universities. The six microscopists met to obtain consensus agreement on the fibers in each designated field classified as countable under National Institute for Occupational Safety and Health Method 7400. Slides and documentation were forwarded to AIHA for training or other purposes. Examination of the results by statistical methods showed that some microscopists’ results were significantly different from others, even though all analysts would have been considered proficient with respect to the final consensus values. Although the reasons for the outliers are complex, this procedure may have value in selecting reference laboratories in proficiency test schemes, possibly leading to more defensible “true” values and tighter limits of variation. © 2003, Taylor & Francis Group, LLC. All rights reserved.