Cyclosporin A blocked production of the lymphokine interleukin 2 by activated T lymphocytes. In a human and a murine [mouse] cell line this inhibition reflected an absence of interleukin 2 mRNA. Under conditions in which these cells are normally stimulated to secrete high levels of interleukin 2, they failed to do so in the presence of cyclosporin A. In both cell lines this failure was accompanied by an absence of interleukin 2 messenger accumulation. [The use of this drug as an immunosuppressant in humans was outlined.].