The role of gut-derived CD4+ T cells in clearance of Pseudomonas aeruginosa from the lung was studied by cell transfer experiments. Mesenteric lymph node cells from unimmunized rats, or rats orally immunized with either killed P. aeruginosa or Helicobacter pylori, were transferred to naive rats which were subsequently challenged intra-tracheally with live P. aeruginosa. Recipients of unseparated mesenteric lymph node cells, purifed T cells or CD4+ T cells, from P. aeruginosa-immunized donors, all exhibited enhanced bacterial clearance from the airways compared to recipients of cells from unimmunized donors. Enhanced clearance by T cells was antigen-specific as no enhanced clearance was observed by transfer of cells from donors immunized with H. pylori.