A patient with constitutional Fanconi-like aplastic anemia who developed a preleukemic state was transplanted with bone marrow from his HLA-identical MLC-negative brother. Before transplantation his mononuclear leukocytes were found to have a defective stimulating and responder capacity in mixed leukocyte culture with cells of an unrelated individual. After removal of T cells the stimulating capacity was completely restored. MLC testing with separated cell fractions seems important to establish HLA-D identity with a possible donor, especially when MLC tests with unseparated cells are negative.