Extracts of human or calf thymus influence differentiation of human bone marrow cells in vitro. Incubation of a putative stem cell fraction of human marrow with extracts of thymus for a period of 2 hr led to the appearance of lymphocytes with surface antigens recognized by a highly specific anti-T-cell antiserum. To a lesser degree, development of lymphocytes having the capacity to form rosettes with sheep erythrocytes was observed. Treatment of the so-called stem cell fraction with thymic extracts did not yield cells responsive to mitogenic influences of phytohaemagglutinin or allogeneic cells. This model should permit further analyses of immunodeficiency diseases and provide a useful technique for purification and analysis of thymus extracts active with human haematopoietic cells.