• Twenty-four children receiving maintenance chemotherapy for acute lymphoblastic leukemia were given booster doses of tetanus-diphtheria combined toxoids. One month later, 19 of the 24 children were given Haemophilus influenzae B oligosaccharide-cross-reacting material conjugate vaccine. Following immunization, all patients had protective antibody titers against tetanus, 92% had protective antidiphtheria titers, and 84% had protective titers against H influenzae. Preimmunization titers, postimmunization titers, and response to immunization varied according to the intensity of therapy. There was no correlation with duration of therapy or quantitative hematologie values in the peripheral blood. These observations support the recommendation that children treated for acute lymphoblastic leukemia should be immunized against H influenzae B.