We investigated the adoptive transfer of immunity to herpes simplex virus (HSV) in 61 recipients of T-cell-depleted marrow allografts. Up to 3 months after bone marrow transplantation (BMT), high titres of HSV antibody (Ab) are passively acquired from blood products. This antibody has a half-life of 40.3 days, so that determination of recipient immunity to HSV cannot be made during the early post-transplant period. By 4 months after the allograft, seronegative recipients of seronegative donors had near-background levels of HSV Ab. Seropositive recipients, whether they were excreting virus or not, had high titres of HSV Ab regardless of donor status. Surprisingly, the seronegative recipients of HSV seropositive donors remained seronegative, a result at variance with observations made in the recipients allografts from which T-cells had not been removed. Thus, T-cell depletion modifies the adoptive transfer of humoral immunity to HSV, and an immune donor alone no longer ensures continued antibody production in the recipient.