At a consultation arranged by the World Health Organization's Regional Office for Europe in 1980 a reference method was specified for sampling and evaluating airborne dust samples collected in man-made mineral fiber (MMMF) workplaces. This method involves sampling onto a membrane filter and counting the fibers with the use of a phase contrast optical microscope (PCOM). The evaluation procedure has been tested by members of the WHO/EURO MMMF technical committee through a series of interlaboratory sample exchanges. The results from these trials have shown a consistent improvement in the reproducibility of MMMF counts, maximum systematic interlaboratory differences being reduced from 2.5 times to 1.4 times. This reduction has been achieved through harmonization of the subjective judgements made by microscopists. As this improved agreement was achieved, each laboratory's counting level changed; preliminary comparisons suggest that this occurrence would produce an approximately two- to threefold increase in the fiber concentrations reported during the epidemiology study of the Joint European Medical Research Board. Further work is currently being undertaken to quantify this change better. A reference method for sizing and counting airborne MMMF with a scanning electron microscope (SEM) has also been developed, and on the basis of preliminary experimental tests there has been some harmonization between the participating laboratories.