The U. S. Environmental Protection Agency (EPA) is required, under the amended Clean Air Act of 1977, to review the scientific basis for the total suspended par-ticulate (TSP) ambient air quality standard and determine whether a revised particulate standard can be promulgated by December, 1980. It is recommended that research to develop information for a size-specific standard should focus on inhalable particulate (IP) matter defined as airborne particles ≤15 jum aerodynamic equivalent diameter. This particle size range relates to that fraction of particulate matter which can primarily deposit in the conducting airways and the gas-exchange areas of the human respiratory system during mouth breathing. It is also recommended that a second particle size cut-point of ≤2.5 yum diameter be incorporated in the air sampling devices, based upon considerations of the chemical composition and the size distribution of airborne particles, and on the predominant penetration of particles ≤2.5 /im diameter into the gas-exchange region of the respiratory tract. Data collected in this size range could be used in conjunction with epidemiological health parameters to refine an inhalable particulate standard in the future.