Application of solid manure and manure slurry to land represents an important source of antibiotic compound release to the environment.  High-use veterinary antibiotics from the tetracycline and fluoroquinolone classes are characterized by chemical structures with charged polar functional groups that may participate in ion exchange and ligand exchange interactions with solid environmental phases.  Earlier studies of organic matter sorption indicated that, while hydrophobic interactions are unimportant, metal-bridged complexes between antibiotics and organic matter ligand groups (i.e., antibiotic-cation-organic matter) could be an important mechanism for antibiotic association with manure solids.  Furthermore, the composition of manure slurry, including the abundance of other cations and competing ligands, would affect whether antibiotics are associated with solid or liquid phases when land application occurs.  Results from investigations of the effects of manure composition on the mobility of tetracycline and fluoroquinolone antibiotics in soils following manure application will be presented.