Forty-five adult allogeneic BMT recipients were randomized to receive Haemophilus influenzae type b (Hib)-diphtheria toxoid conjugate vaccine (PRP-D) at 6 and pneumococcal polysaccharide vaccine (Pnc PS) at 8 months (early group; n = 23) or at 18 and 20 months (late group; n = 22) after BMT, respectively. Serum antibody concentrations against Hib and Pnc PS types with varying immunogenicity (serotypes 3, 6B, and 19F) were measured by ELISA. The responses at 1 month after vaccination with PRP-D and Pnc PS were poor and similar in the two vaccination groups, except that two-fold responses in the concentration of antibodies to the most immunogenic Pnc serotype 3 occurred more frequently in the late group. In the late group recipients, the antibody responses elicited by Pnc serotypes 3 and 19F correlated with the concentrations of the corresponding antibodies in the donor. This study on allogeneic BMT recipients shows that vaccination at 6-8 months after BMT with one dose of PRP-D and Pnc PS vaccine is as immunogenic as vaccination at 18-20 months after BMT, and suggests that donor immunity to Pnc PS affects recipient responses to Pnc PS vaccination. Consequently, as among allogeneic BMT recipients the greatest risk for infections by encapsulated bacteria occurs during the first 1-2 years after BMT, Hib and Pnc vaccines should not be given later than 6-8 months post-BMT.