The effect of the polydispersity of fibrous materials on the viscous drag and the efficiency of gas filtration has been studied. The dependence of the Brinkman constant on parameter beta(g), which characterizes the variance of the lognormal function of fiber-length distribution over fiber radii, has been calculated for equations describing the hydrodynamics of a gas in a porous medium. The dependences of the effective aerosol- particle-collection efficiency on the packing density, dispersion of fiber-size scatter, and average fiber radius have been investigated. Quality parameters of filters with different degrees of polydispersity have been compared. Results of calculations have been compared with experimental data.