In vitro proliferation of T lymphocytes of thymic versus extrathymic origin. Spleen T cells from B6 mice (thymic T cells) and from irradiated adult-thymectomized B6.SJL (Ly5.1) hosts reconstituted with LckOM fetal liver cells (Ly5.2; extrathymic T cells) were labeled with CFSE and stimulated with anti-CD3 with or without anti-CD28. After culturing for 24 to 96 hours, cells were labeled with 7-AAD to exclude dead cells and stained with antibodies against CD4, CD8, Ly5.1, and Ly5.2. Total cell numbers are expressed as a function of X, which represents the number of cells per well at 24 hours (before the beginning of cell division). This is one representative experiment of 5 (Table 1).