Clearance of tagged heavy metal aerosols (Fe, Hg, Mn, Ba, and U) in female beagles by in vivo counting was assessed. Fecal excretion paralleled lower respiratory tract clearance of Fe. No Fe was found in blood, indicating its biological insolubility. Ultrafiltration tests (an indication of biological solubility) ranked the heavy metals as: Hg, Mn, Fe, and U in order of decreasing solubility. The solubility ratios were remarkably similar to the ratios of inverse biological half-times.