The chelation-mediated mobilization of 239 Pu and59 Fe from rat alveolar macrophages which had engulfed239 Pu-59 Fe-hydroxide colloids was studied by exposing the macrophages to diethylenetriaminepentaacetic acid and macromolecular weight forms of diethylenetriaminepentaacetic acid. The release of239 Pu and59 Fe by these macromolecular weight chelating agents and diethylenetriaminepentaacetic acid was assessed in monolayer culture and shown to be dependent upon the nature of the engulfed colloids. Whereas calciumdiethylenetriaminepentaacetic acid, aminoethylcellulose-diethylenetriaminepentaacetic acid, and aminoethylcellulose-diethylenetriaminepentaacetic acid oxidized with periodic acid (aminoethylcellulose-diethylenetriaminepentaacetic acid(I2 ) exhibited some effectiveness at releasing239 Pu and59 Fe from macrophages which had engulfed the low aggregative colloid, they were less effective in the release of these radionuclides from the high aggregative colloid. The efficacy of these chelating agents on the removal of radionuclides from alveolar macrophages differed between iron and plutonium, suggesting the significant difference in the dissolution and subsequent metabolism of these radionuclides in alveolar macrophages. From the similarities of the percentage increases in239 Pu mobilized from the low aggregate colloid by aminoethylcellulose-diethylenetriaminepentaacetic acid (I2 ) and diethylenetriaminepentaacetic acid, it would appear that both forms of diethylenetriaminepentaacetic acid could be accessing the same deposits of239 Pu; however, for59 Fe the percentage increases are quite different for the two forms of diethylenetriaminepentaacetic acid. SiO2 -diethylenetriaminepentaacetic acid, an insoluble macromolecular weight chelating agent, was ineffective at releasing239 Pu but was slightly effective at releasing radioiron.