Allospecific suppressor T cells can be generated by allostimulating thymus, spleen and lymph node cells in culture. Bone marrow cells do not yield suppressor lymphocytes. While the efficiency of thymocytes for suppressor cell generation decreases with the age of thymus donors, the efficiency of spleen increases with age. Generation of suppressor lymphocytes depends on cell replication and protein synthesis. The suppressor function as such does not require cell replication. Induction of suppressor T cells in culture requires cell to cell interaction of cortical type and medullar type T cells.