Use of anti-CD20 monoclonal antibody after allogeneic HCT for the treatment EBV lymphoma or viremia, autoimmune cytopenias, and the prevention or treatment of recurrent NHL has become more frequent. There is limited data on the effect of this therapy on specific antibody production following allogeneic HCT. We therefore examined the response to standard childhood immunizations of 30 patients, including 20 adults (>18 years of age) who received rituximab following a T cell depleted (n=26) or unmodified (n=4) HCT at our institution. T cell function was assessed in vitro by proliferative response to PHA and B cell reconstitution was confirmed by recovery of CD19+ and CD20+ circulating lymphocytes. Patients received a median (range) of 4 (1–12) doses of rituximab, with 15 patients receiving 4–6 doses. The median age of the patient population was 28.5 years with a range of 0.8–63.0 years. Children (median age: 8.5 yrs) received a transplant from an unrelated (n=7), HLA mismatched related (n=2), or HLA matched related (n=1) donor and adults (median age: 42.5 yrs), received a transplant from an unrelated (n=7), HLA mismatched related (n=2), or HLA-matched related (n=11) donor. Patients were vaccinated when their PHA response was at least 75% of the lower limit of normal and CD20+ cells were >100/ul. Vaccination was initiated at a median of 270 and 525 days following the last dose of rituximab in children and adults, respectively. A two to 3-fold rise in titers following a series of 3 tetanus and IPV vaccinations occurred in 19 of 21 and 19 of 19 evaluable patients, respectively. Although 8 of 9 children responded to the H flu conjugate vaccine, only 5 of 13 adults mounted an adequate response. Eighteen patients received the recombinant Hepatitis B vaccine. Nine responded following the initial series (5/7 children, 4/11 adults). Two of three patients who failed initial Hepatitis B vaccination, responded to a second series (1 child, 1 adult). Response to pneumococcal vaccination was the least consistent. None of 11 adults who received the unconjugated 23-valent pneumococcal vaccine developed an adequate response. In view of this observation, children given rituximab were immunized with the conjugated pneumococcal vaccine. Four of eight children responded to the initial series, and two additional children responded following a second series. These preliminary data suggest that both adult and pediatric patients given rituximab following an allogeneic HCT can mount successful antibody responses to standard T-helper cell dependent vaccines. However, B cell responses to T cell independent vaccines may be particularly impaired. Larger prospective trials, stratified by age, comparing vaccine responses in patients following allogeneic transplant in the presence or absence of post transplant rituximab therapy are warranted.