in vitro t cellmediated killing of pseudomonas aeruginosa iv nonresponsiveness in polysaccharideimmunized balbc mice is attributable to vinblastinesensitive suppressor t cells we reported previously that balbc mice immunized with a polysaccharide ps antigen isolated from immunotype 1 pseudomonas aeruginosa and vinblastine sulfate develop t cellmediated protective immunity despite their failure to produce specific antibody in vitro lyt12 ij t cells from vinblastine and psimmunized mice kill p aeruginosa by secretion of a bactericidal lymphokine balbc mice immunized with ps alone generate neither protective antibodies nor a protective t cell response the current studies indicate that t cells from mice immunized with ps alone significantly suppress the bactericidal activity of t cells from mice immunized with vinblastine and ps the suppressor t cells are of the same lyt12 ij phenotype as the bactericidal t cells suppression is mediated by a soluble product of these suppressor t cells which both inhibits t cell proliferation and interferes with the production or release of the bactericidal lymphokine cyclophosphamide used in other systems to remove suppressor t cells fails to enhance bacterial killing and does not inhibit suppressor cell activity these studies indicate that immunization with ps elicits responses in two functionally distinct subgroups of lyt12 ij t cells and that these cells are distinguishable by their sensitivity to vinblastine sulfate