Three different two-stage samplers were calibrated using monodisperse aerosols produced by a spinning-disk aerosol generator. One, using a horizontal elutriator as the size selector, agreed with the criteria defining “respirable dust” adopted by the British Medical Research Council. The other two, using small cyclones as size selectors, agree with criteria proposed by the Los Alamos Conference and the ACGIH when operated at approximately half their initially recommended flow rates. Operation of these cyclones at the initially recommended flow rates underestimates the inhalation hazard.