Frozen sections from bone marrow trephine biopsy specimens from children with disseminated neuroblastoma were stained using the monoclonal antibody UJ13A. An immunoalkaline phosphatase technique was the preferred staining method. Acceptable histological detail was obtained from this material and deposits of tumour cells detected. Some apparently fibrous tissue was also stained by this antibody. The results show that this immunohistological approach is feasible and provide encouragement for its addition to the range of investigations currently available for assessing the marrows of children with this disease.