The in vitro measurement of stimulated T lymphocyte blastogenesis by the incorporation of 14C-thymidine can be shown to be an acceptable and precise measurement with highly significant variations in blastogenesis over a 24 hr period. This circadian rhythm for human T lymphocyte blastogenesis has been demonstrated with specific antigen (SKSD) and in mixed leukocyte culture (MLC). The hours of peak reactivity were different for the SKSD stimulation and the MLC reaction. This biologic cycle in T cell reactivity has significant implications for the immunologic response.