Inclusion Criteria:  An underlying hematological malignancy including:  a) relapse of AML after allogeneic hematopoietic stem cell transplantation; b) relapse of ALL after allogeneic hematopoietic stem cell transplantation; c) relapse of JMML after allogeneic hematopoietic stem cell transplantation; e) refractory ALL; f) refractory AML; g) high risk infant ALL;  be ≥ 0.5 year old and ≤ 18 years old; must not be eligible for therapy of higher curative potential. Where an alternative therapy has been shown to prolong survival in an analogous population, this should be offered to the patient prior to discussing this study; have a Karnofsky Performance Status ≥ 50 or Lansky Performance Status ≥ 30; provide signed, written informed consent from parent or guardian; be able to comply with study procedures and follow-up examinations; have adequate cardiac function (irrespective of concomitant cardio-vascular treatment) at PI/CI discretion; have adequate organ function (as indicated by Table 5) within 30 days prior to 111In infusion; patients who have received any other chemotherapy within the previous 2 weeks and must have recovered from acute toxicity of all previous therapy prior to enrolment; be negative for human-anti-murine antibodies (HAMA).  Exclusion Criteria:  patients who are positive for human anti-murine antibodies (HAMA); patients with compromised organ function within 30 days prior to 111In infusion; patients with isolated CNS disease relapse*; patients with an active, uncontrolled systemic infection considered opportunistic, life threatening, or clinically significant at the time of treatment; Pregnant or breast-feeding females are excluded due to potential risks of foetal adverse events of an investigational agent. Pregnancy tests must be obtained prior to enrolment on this study for girls of reproductive potential. The need to commence pregnancy testing will be at the discretion of the treating physician to facilitate taking in to account factors such as precocious puberty, endocrine status and medications which can affect pubertal status. Males or females of reproductive potential may not participate unless they have agreed to use an effective contraceptive method. Sexual Abstinence is an acceptable method of birth control**. patients with any other severe concurrent disease, which, in the judgment of the Investigator, would make the patient inappropriate for entry into this study; patients with extensive chronic graft versus host disease (GVHD); patients with unstable cardio-vascular disease. -