The Francisella tularensis T-lymphocyte antigens, which may have a role in protection against tularemia, were investigated with vaccine-immunized subjects. Preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to fractionate the bacterial envelope preparation. The 23 fractions obtained represented membrane proteins of different apparent molecular masses ranging from 10 to 150 kilodaltons. Different fractions contained one to four separate protein bands stained with Coomassie blue. The lymphocyte blast transformation responses of five tularemia vaccine-immunized and three nonimmunized subjects were tested against bacterial material eluted out of SDS-PAGE fractions. Every fraction stimulated lymphocytes from at least one of the subjects. No clearly immunodominant or inhibitory antigens were detected among the envelope fractions. Expression of the HLA-DR antigen at the surface of CD4- and CD8-positive lymphocytes was also studied as a measure of cell activation. The numbers of CD4+ DR+ cells varied directly with the lymphocyte proliferation profiles, and very few CD8+ cells were found in the preparations stimulated with the different fractions. The diversity of the antigens recognized by immune T lymphocytes was confirmed by using F. tularensis-specific T-lymphocyte clones obtained from vaccinated subjects. Most of the 36 T-lymphocyte clones tested were stimulated by one SDS-PAGE fraction only.