Fly-ash particles fractionated into three size ranges (<2, 2 to 5, and 5 to 8 micrometers) and coated with various metal oxides were used to determine whether particle size and surface area are contributing factors to the in vitro toxicity of trace metals for alveolar macrophages (AM). Within a given particle type and exposure concentration, decrease in particle size resulted in decreased viability and increased phagocytosis. The toxic effect was not due to the solubilization of the test elements into the incubating medium. The percentage of lead, nickel, or manganese adsorbed on the fly-ash particles varied within a relatively narrow range and was not affected by the particle size. Since the AM were exposed to the particles on a weight-dose per cell basis, their effective exposure load to the test elements was constant irrespective of the particle size. Thus the greater toxicity of the small particles appears to be due to surface interaction between particles and AM, and toxic effect is surface-area as well as dose related.