The protective effect of primed CD4+ T lymphocytes against a lethal dose of 10(8) viable Salmonella typhimurium was studied in Lewis rats. Primed CD4+ T lymphocytes were obtained by inoculating Lewis rats with a non-lethal dose of 10(6) viable S. typhimurium. Four weeks after the infection, spleen CD4+ T lymphocytes were separated using magnetic microspheres coated with an antibody against the CD4 molecule (W3/25). Subsequent sorting into activated and non-activated subpopulations using the p55 alpha-chain of the interleukin-2 receptor (CD25) as an activation marker was performed by a fluorescence-activated cell sorter. Untreated Lewis rats were injected with 10(4) different primed CD4+ T-cell populations 24 h prior to the lethal dose of 10(8) viable S. typhimurium. Blood samples were drawn from the orbital plexus 1, 2, 3, and 4 weeks after the infection, and analysed for specific IgM and IgG antibodies. Cell sorting revealed that 2/3 of the primed CD4+ T lymphocytes expressed high levels of CD25. Cell transfer revealed that both CD25high and CD25low expression populations could induce immunity against a lethal dose of S. typhimurium, whilst antibody analysis revealed that antibody levels were not correlated with protection against S. typhimurium infections, although it showed that a higher and more persistent level of specific IgG antibodies was produced in animals receiving the CD4+CD25high fraction. It is concluded that 10(4) primed CD4+ T lymphocytes can induce immunity in animals challenged with a lethal dose of S. typhimurium and that antibodies do not seem to be correlated with the immunity induced. The CD4+CD25high fraction was, however, associated with a higher and more persistent level of specific IgG antibodies.