This report summarizes some of our past and present studies on T cells and hemopoietic progenitors obtained from patients with severe aplastic anemia (SAA). Two main issues are discussed: enhancement of colony growth by manipulation of accessory cells, or by the addition of patients' plasma, and the role of suppressor T cells in the pathogenesis of the disease and response to antilymphocyte globulin (ALG). A complex network of interactions exists. The identification of cells and soluble factors capable of enhancing or suppressing colony formation promises to help us better understand the events leading first to the development of aplasia and then eventually to hematologic recovery.