Inclusion criteria:

          1. An underlying haematological malignancy including:

               1. isolated bone marrow relapse of AML after allogeneic haematopoietic stem cell
                  transplantation;

               2. isolated bone marrow relapse of ALL after allogeneic haematopoietic stem cell
                  transplantation;

               3. bone marrow relapse of JMML after allogeneic haematopoietic stem cell
                  transplantation;

               4. refractory AML (5-20% blasts in BM) with/without expression of CD66 on blasts;

               5. refractory ALL (5-20% blasts in BM) with/without expression of CD66 on blasts;

               6. refractory AML (> 20% blasts in BM) with expression of CD66 on blasts;

               7. refractory ALL (> 20% blasts in BM) with expression of CD66 on blasts;

          2. be ≥ 1 year old and ≤ 18 years old;

          3. 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;

          4. have a Karnofsky Performance Status ≥ 50 or Lansky Performance Status ≥ 30;

          5. provide signed, written informed consent from parent or guardian;

          6. be able to comply with study procedures and follow-up examinations;

          7. have normal cardiac function without specific treatment;

          8. have adequate organ function (as indicated by Table 3, chapter 5), within 30 days
             prior to 111In infusion;

          9. 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;

         10. be negative for human-anti-murine antibodies (HAMA). Exclusion criteria

        1) patients with CNS disease; 2) patients with BM cellularity < 10%; 3) patients with ≥ 20%
        blasts in BM and no expression of CD66 on blasts; 4) patients who are positive for
        human-anti-murine antibodies (HAMA); 5) patients with compromised organ function (as
        indicated by Table 3, chapter 5), within 30 days prior to 111In infusion; 6) patients with
        extensive chronic GvHD; 7) patients with an active, uncontrolled systemic infection
        considered opportunistic, life threatening, or clinically significant at the time of
        treatment; 8) patients who are pregnant or lactating; 9) patients with any other severe
        concurrent disease, which, in the judgment of the Investigator, would make the patient
        inappropriate for entry into this study.