We have recently discovered that conditioned medium (CM) of phytohemagglutinin stimulated lymphocyte is capable of inducing precursor cells to differentiate and develop to mature T cells. This was seen with the development of markers and functions on precursor leukemic null lymphocytes (lacking the usual B and T cell markers) upon culture with CM. The differentiation assay allows also growth of T cells, thus enabling us to delineate T cell development. Cell growth can be quantitated by a microassay with incorporation of radiolabeled thymidine. Employing these differentiation and proliferation assays, we plan to analyze T cell development in leukemias. The potential for differentiation and development of patient cells in culture induced by CM will be analyzed. We will determine the presence or absence of inducible T cell precursor, thereby defining the disease. The sequence of T cell development from precursor to subclasses, as assessed by membrane markers and functions will be studied in correlation with chromosomal analysis. Comparing the normal and patient T cell developmental sequence, we expect to learn the abnormalities in leukemias. The differentiation block in patients will be analyzed. Patient or normal CM will be cross-tested with normal or patient cells. An assessment will be made as to whether there is a lack of CM production or a block between CM and the inducible precursor cells. The potential for proliferation of patient cells by CM will be analyzed. Sera of leukemic patients will be analyzed in the differentiation and proliferation assays for any abnormalities in regulating T cell development. The interactions between precursor cells, serum and differentiation factor will be analyzed to understand abnormalities of the disease. We also aim at better recognition of the differentiation factor at the molecular level. This important and fundamental information will facilitate a better definition of the leukemias and make a major contribution to the understanding of the regulation of hematopoiesis and the immune system.