Development and evaluation of new vaccines and immunotherapy against tuberculosis demand a better understanding of the immune mechanisms in this disease. Costimulatory signals and intercellular contact seem to be pivotal in determining whether recognition of antigen by T cells leads to activation or anergy. In this paper, we show that virulent M. tuberculosis H37Rv downmodulates the ex vivo expression of CD18 and CD86 on peritoneal macrophages and VLA-4 on lymphocytes but does not disturb the in vitro production of interleukin (IL)-12 and interferon (IFN)-γ after intraperitoneal infection. In addition, splenocytes from infected mice produce IL-10, while the expression of cell surface receptors is unchanged. The interplay among IL-12, IFN-γ and IL-10 in vivo and the downmodulation of cell-surface receptors during the infection at the inflammatory site may contribute to the explanation of the maintenance of infection.