Forty-seven patients (age range, 7 months-18 years) with malignant (38 cases) and nonmalignant (9 cases) disorders given an allogeneic or an autologous bone marrow transplantation (BMT) were immunized with Haemophilus influenzae type b (Hib) polysaccharide-diphtheria toxoid conjugate vaccine administered in a single dose at different time points after transplantation. Results were compared with those of 13 healthy children matched for age and sex who received the same immunization schedule. Serum and saliva samples for measurement of total IgG subclass and specific antibody levels were obtained from patients and healthy controls before and 3 weeks after vaccination. Twenty-five of the 47 patients (53%) had a specific anti-Hib IgG response, while an effective IgA and IgM response was mounted by 23 (49%) and 11 (23%) children, respectively. In the control group, 13 of 13 subjects mounted a specific IgG antibody production (P < 0.005 in comparison to the patients' response rate), while an IgA and IgM response was demonstrated in 12 (92%; P < 0.01 compared to transplanted patients) and 7 (54%; P < 0.05 in comparison to BMT recipients) children, respectively. Lapse of time from BMT to immunization was the most important factor predicting antibody response, as proved by an effective increase in prevaccination specific IgG levels in the majority of patients vaccinated after 2 years from transplant. Our data demonstrate that BMT recipients have a reduced capacity to mount an antibody response to polysaccharide antigens compared to normal controls, even when a protein-conjugated vaccine is employed. Since time after transplant is the major factor influencing the recovery of immune reactivity to polysaccharide antigens, the ontogeny of the B cell repertoire seems to follow a predetermined sequential program of development.