In order to compare methods of counting asbestos fibers in lung tissue, seven laboratories participated in an interlaboratory trial in which tissue samples from five human lungs were analyzed. In some laboratories, fiber concentrations were assessed with the light microscope and, in others, with either scanning or transmission electron microscopes. Within each laboratory the ranking of the results was similar, but there were marked differences in the absolute values obtained by the different laboratories. It is concluded that the laboratories participating in this trial appear to produce internally consistent results, but there is difficulty in directly comparing results from one laboratory to the next.