In a randomized study, 20 adult allogeneic BMT recipients were vaccinated at 6 months and 22 at 18 months after BMT with Haemophilus influenzae type b (Hib)-diphtheria toxoid conjugate vaccine (PRP-D), and 23 recipients at 8 months and 21 at 20 months with pneumococcal polysaccharide (Pnc PS) vaccine. IgG1 and IgG2 subclasses of Pnc PS and Hib antibodies and avidities of Pnc PS IgG antibodies were determined by EIA in sera from patients with at least a two-fold total antibody response to Pnc type 3, 6B, 19F or PRP-D. The Pnc PS vaccine induced predominantly IgG1 Pnc 3 antibody production. Anti-Pnc 6B and 19F responses were mainly IgG2. The time of the Pnc PS vaccination, at 8 or 20 months after BMT, did not influence the IgG subclass response pattern. The PRP-D vaccine induced predominantly IgG2 anti-Hib production in the patients vaccinated at 6 months after BMT. The patients vaccinated at 18 months produced IgG1 and IgG2 antibodies more evenly. The same patient was able to produce predominantly IgG1 subclass antibodies to one antigen, Pnc 3, 6B, 19F or Hib, and IgG2 antibodies to another. The avidities of anti-Pnc 6B and 19F 1 month after vaccination were similar to those before vaccination, anti-Pnc 3 avidity was lower than before vaccination but matured in 15 months. The IgG subclass distribution and avidity were similar in the patients with and without chronic GVHD. In conclusion, the IgG response to Pnc type 3 was predominantly IgG1 as in infants and IgG2 to PRP-D, Pnc 6B, and 19F as in adults. Early vaccination after BMT or the presence of chronic GVHD did not impair the quality of response to Pnc PS and PRP-D vaccines.