Recovery from influenza virus infection is dependent on T cell functions which can be provided either by CD8 or CD4 T cells. To identity the functions involved in recovery promoted by CD4 T cells, we have studied the course of the infection in B-cell deficient micro MT mice which had been depleted of CD8 T cells by antibody treatment. Upon infection with PR8 [A/PR/8/34(H1N1)], such B- and CD8 T cell-deficient mice mounted strong CD4 T cell responses that were comparable in size and cytokine secretion to those seen in intact mice. Yet, these B- and CD8 T cell-deficient mice could not clear the infection, in contrast to (CD8-depleted) mice containing both B- and CD4 T cells. These findings indicate that the promotion of the T-dependent antibody response is an indispensable component in the CD4 T cell-dependent recovery process.