Advantages and disadvantages of the estimation of the mass and number fibrous dusts concentrations are discussed. By using the reference phase contrast microscopic method (1) the data of interlaboratory comparative measurements of MMMF (man-made mineral fibres/density/f.mm-2) have been got and are presented. Total mass dust concentrations and number concentrations of MMMF and/or asbestos fibres are compared. The consequent measurements were made in the same time intervals and in the same workplaces. In conclusion both measuring methods are critically evaluated and their results are compared with the prescribed MAC-values. Presented results are of use when deciding what method with respect to the kind of fibrous dust and to the working operation should be chosen.