Immunoglobulin class- and subclass-specific antibodies to a polyvalent pneumococcal capsular polysaccharide vaccine (Pneumovax II) were measured before and after immunization in children, 1 year or more after bone marrow transplantation for a variety of genetic disorders. The median titres of specific IgG, IgG1 and IgG2 pneumococcal antibodies fell significantly (P less than 0.05) from pre-transplantation levels. The levels of pneumococcal antibodies in the patients before immunization were markedly lower than those in control children of comparable age, for antibodies of IgM, IgG, IgG1 and IgG2 classes (P = less than 0.001 in each case). Apart from IgG2 antibodies, the median response to immunization with Pneumovax II was not significantly different from the controls (P greater than 0.05). However, because of the lower pre-immunization levels, the patients did not achieve a high post-immunization-specific antibody titre in any immunoglobulin class or subclass, when compared with normal children. Neither the pre-immunization specific antibody levels nor the response to immunization were affected by splenectomy or the presence of chronic graft-versus-host disease. Immunization of the donor before bone marrow harvest did not influence the level of specific antibody 1 year or more after transplantation. No significant correlation was found between the total serum IgG2, the patients' age at the time of assessment, or time after transplantation, and the IgG2-specific antibody response. The lack of specific antibodies and the poor IgG2 response to pneumococcal antigens may contribute towards the occurrence of infection with Streptococcus pneumoniae in the late post-transplantation period.