Abstract: Two monoclonal antibodies (3-3 and 3-40) were produced which identify two new leukemia associated antigens. Both antibodies reacted with most cell lines derived from patients with T lymphoblastic leukemia (T-ALL), but were not detected on suspensions of normal hematopoietic cells, including thymocytes, by cytotoxicity, absorption of indirect immunofluorescence assays. Analysis of fresh leukemic cells indicated that mAb 3-3 only reacted with T-ALL cells, while mAb 3-40 reacted with some non-T non-B ALL cells and a few acute myelocytic leukemia (AML) cells, as well as T-ALL cells. The 3-40 antigen was also found histopathologically in frozen sections of several normal tissues, including the epithelial cells and a few lymphoid cells of the thymus, and some malignant tissues, while the 3-3 antigen was not found in any tissue studied. A "double absorption" assay provided additional serological evidence the the two antibodies identify different antigenic determinants.