Monoclonal antibodies 3A1, 4F2, 5E9, OKT3, OKT6, and OKT8 were tested by flow microfluorometry for reactivity with cells from patients with T-cell or null cell acute lymphocytic leukemia (ALL). The 3A1 antibody reacted with a greater percentage of cells from 1 group of patients that could be classified as T-cell leukemia patients as compared to its reaction in the group classifiable as null cell leukemia patients. Reactivity patterns of other monoclonal antibodies with lymphocytes from patients with ALL did not clearly define groups of ALL patients. All cells from individual leukemia patients reacted differently with the seven monoclonal reagents. On the basis of patterns of reactivity, T-cell leukemias could not be classified as malignant clones of cells from the previously described stages of thymus cell maturation. The results demonstrated that the cells undergoing malignant change express a variety of combinations of antigens not found on normal lymphocytes.