Dinitrophenylation of Salmonella typhimurium bacteria increased their hydrophobicity, negative charge, and interaction with PMNL in vitro, effects which are similar to the consequences of S-R mutation in S. typhimurium. The effects are ascribed to the hydrophobicity of the dinitrophenyl (DNP) ligand. Coupling of DNP-groups to the bacteria also enhanced their clearance by the reticuloendothelial system after intravenous injection into mice. Extensive DNP-coupling to the bacteria led to rapid deposition of injected bacteria outside the reticuloendothelial system. The kinetics of DNP-coupling showed saturation at 4-5 X 10(7) DNP groups per bacterium, similar for S and R bacteria, boiled and nonboiled. After 2 h of reaction with fluoro-2,4-dinitrobenzene (FDNB) more than 80% of the total DNP coupling observed after 4 h had occurred. The capacity of MOPC-315 IgA myeloma protein, which possesses anti-DNP activity, to bind to dinitrophenylated bacteria paralleled the number of DNP groups per bacterium, but smaller numbers of IgA molecules were bound. Approximately one IgA molecule bound per thousand DNP ligands. Only when bacteria were boiled prior to DNP coupling was agglutination observed after the addition of anti-DNP-IgA.