Virulent strains of Pseudomonas aeruginosa derive their pathogenicity, in part, from their secretion of two proteolytic enzymes, alkaline protease (AP) and elastase (E). Human T lymphocytes specific for AP and E can be detected in the blood of immune donors and have afforded the opportunity to characterize the antigenicity of these proteins. To accomplish this goal, we have recently selected 68 human T cell clones from five different Pseudomonas-immune donors and determined their fine specificities. Fifty-five (81%) were found to be protease specific, demonstrating the immunogenicity of the exoenzymes in humans. These clones defined five AP and three E specificities and suggested the existence of at least five allomorphic determinants expressed on the proteases of various Pseudomonas strains. Limiting dilution analysis confirmed a number of antigenic relationships suggested by the long-term T cell clones and revealed that T cells specific for allomorphic protease determinants were at least as frequent in the blood of immune donors as were T cells committed to conserved determinants. Thus, both primary and long-term human T cell clones showed specificity patterns that distinguished proteases from different Pseudomonas strains. These observations describe a heretofore unknown antigenic system of Pseudomonas aeruginosa that should assist in defining the nature and specificity of Pseudomonas immunity in humans.