There are presently no monitoring tools to assess the immunosuppressive effect of cyclosporine (CsA) in vivo, since the mode of drug action is incompletely understood in man. Experimental in vitro studies suggest that CsA causes reversible inhibition of T helper cell generation of interleukin-2 (IL-2). Therefore the present study examined the effect of CsA administered in vivo on the capacity of kidney transplant recipient lymphocytes to generate IL-2 after mitogen (phytohemagglutinin [PHA]) stimulation. IL-2 production was measured by the capacity of lymphocyte supernates to trigger proliferation of a human IL-2-dependent T cell line. Peripheral blood lymphocytes (PBL) from CsA-Pred treated recipients displayed 40.6% inhibition (1.14pm0.06 U/ml, n=117, P