MOST Of the T lymphocytes that populate the immune system develop in the thymus before its involution during late adolescence, Therefore, subsequent losses in T cells caused by HIV infection(1), chemotherapy(2) or age related factors(3) can greatly diminish immune responses to new antigenic challenge, Here we report the discovery of a thymus-independent pathway of T-cell development that may provide help for T cell immunodeficiency, We show that expression of an oncostatin M transgene(4) in the early T lineage stimulates a dramatic accumulation of immature and mature T cells in lymph nodes, A functional thymus is not required for this effect as reconstitution of nu/nu mice with transgenic bone marrow stimulated a 500-fold increase in Thy-1(+) lymph node cells and restored immune responsiveness to allogeneic mouse melanoma cells, This lymphopoietic pathway is not unique to transgenic mice because administration of oncostatin M protein produced a similar response in non-transgenic mice, These results identify a new pathway of T-cell development and a potential treatment for T-cell immunodeficiency with oncostatin M.