OBJECTIVES: I. Assess the efficacy and toxicity of iodine I 131 monoclonal antibody BC8, cyclophosphamide, and total body irradiation in patients with advanced acute lymphocytic leukemia who are receiving HLA matched related or unrelated bone marrow transplantation. II. Determine the maximum tolerated dose (MTD) of iodine I 131 monoclonal antibody BC8 in these patients. III. Estimate the MTD of radiation delivered by iodine I 131 monoclonal antibody BC8 to the marrow. IV. Study the influence of marrow cellularity, level of antigen expression by leukemic cells, and degree of antigen saturation by antibody on the biodistribution of iodine I 131 monoclonal antibody BC8 in these patients.