To test the influence of T cell depletion of the marrow in allogeneic bone marrow transplantation on functional T cell recovery, in vitro lymphocyte proliferation tests (LPTs) to microbial antigens were regularly performed in 23 recipients of normal BM and in 25 patients receiving BM with a fixed low number of T cells (1×105T cells/kg body weight; recipients of T-depleted BM).The long-term recovery of positive LPT to at least 1 of the 4 tested microbial antigens—Candida, herpes simplex virus (HSV), varicella-zoster virus (VZV), and cytomegalovirus—was nearly similar in both groups: 16/23 versus 18/25. Recovery of LPT toCandidaand HSV in the first 3 months appeared to be greatly influenced by prophylactic measures; only 2/23 recipients of normal BM, receiving amphotericin B, showed a positive LPT toCandidaversus 13/25 recipients of T-depleted BM (P < 0.01). In contrast, only 1/23 seropositive recipients of T-depleted BM, receiving acyclovir, showed a positive LPT to HSV versus 9/22 recipients of normal BM (P < 0.05). A positive LPT to CMV in the first 3 months was found in 9/9 seropositive recipients of normal BM, versus in 5/11 seropositive recipients of T-depleted BM (P < 0.05). Five of the 6 patients with a negative LPT died of CMV-interstitial pneumonia versus 1/14 with positive LPT (P < 0.01).We conclude that in CMV-seropositive recipients of allogeneic BM, T cell depletion of the graft affects the early recovery of T cell proliferation to CMV, which is associated with a higher risk of fatal CMV-interstitial pneumonia.