Lethally irradiated Balb/c mice reconstituted with syngeneic T cell-depleted or syngeneic untreated bone marrow (BM) were able to produce equal levels of hemagglutination inhibition (HI) antibodies at 4 weeks after bone marrow transplantation (BMT) in response to nasal infection with A/PR8 influenza virus given 1 week after BMT. Likewise, no differences in mortality rates could be observed following influenza virus infection 1 day after BMT. Antibody production was detected in 10-20% of BMT recipients. All animals responded to a secondary infection given 2 months later by production of secondary IgG-type HI antibodies. Mice reconstituted with BM enriched with spleen cells obtained from immune donors showed an improved survival rate as compared with recipients of naïve BM, immune BM or T-depleted BM obtained from immune mice. Our results indicate that T cell depletion by itself does not increase susceptibility of syngeneic BMT recipients to virus infection. However, immune spleen cells may play a significant role in conveying protection against influenza virus infections. Although recipients of both immune and naïve intact BM may mount better anti-influenza titers as compared with T-lymphocyte-depleted BMT recipients, it appears that the proportion of immune donor T cells in the marrow inoculum is insufficient for protection against influenza virus infection. Generation of memory cells to influenza viral antigens in the post-BMT period is not impaired in recipients of T-depleted marrow grafts, suggesting that memory cell precursors are unaffected by the T cell depletion procedure, or else that they were regenerated during the immediate post-BMT period from Thy 1.2-negative precursors.