Foster, W. Michael, Pamela T. Stetkiewicz, and Arthur N. Freed. Retention of soluble ^99mTc-DTPA in the human lung: 24-h postdeposition. J. Appl. Physiol. 82(4): 1378-1382, 1997.Clearance of low-molecular-weight solutes, e.g., radiolabeled chelate diethylenetriaminepentaacetate (DTPA), across epithelial surfaces of distal airways and the lung parenchyma is a broadly used technique to assess epithelial integrity. It has been generally assumed that clearance of solute follows a simple first-order process and that DTPA clearance through the respiratory epithelium and into blood and lymphatic channels is complete within a few hours. Using -camera imaging and a radiolabeled aerosol of ^99mTc-labeled DTPA, we observed in eight healthy subjects lung retention of radioisotope ~24 h postdeposition of the ^99mTc-DTPA. Residual lung retention at the 24-h end point averaged 6.0 ± 1.8 (SD)% of the amount of radioisotope initially deposited in the lung. This suggests that for normal healthy subjects a small amount of the ^99mTc radioisotope, either in a dissociated or chelated form, is nonpermeable or slowly cleared from respiratory tisssues.