Human peripheral blood T cells of normal donors were activated in vitro with autologous adherent cells pulsed with poly(LTyr,LGlu)-poly(DLAla)--poly(LLys) [abbreviated (T,G)-A--L]. The "educated" T cells were tested: (i) for their ability to produce a (T,G)-A--L-specific T cell-replacing factor in the cooperation with B cells for antibody responses in vivo or in vitro and (ii) for their ability to proliferate in the presence of a second stimulus of (T,G)-A--L. Results of screening of 66 donors demonstrated that educated T cells of about 50% of the donors produced an active (T,G)-A--L-specific factor, whereas activated cells of only half of the factor producers were capable of proliferating in the presence of the antigen. Thus, as reported for all other species studied, human individuals differ in their response potential to (T,G)-A--L.