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Methods to quantitate the effects of immunosuppressive drugs on immune reactivity might be helpful for monitoring immunosuppressive treatment. Cyclosporine (CsA) inhibits the induction of cytokine synthesis in T cells, and measurement of interleukin (IL)-2 production might constitute a parameter of this drug's effect.We determined the percentages of CD4+and CD8+lymphocytes producing IL-2 upon stimulation by phorbol myristate acetate and calcium ionophore in whole blood culture, using immunostaining of intracytoplasmatic and membrane markers, followed by multiparameter flow cytometry. A total of 38 clinically stable transplant patients on various immunosuppressive protocols were studied.The percentage of CD4+T cells producing IL-2 was strongly reduced in patients compared with healthy controls (23%[range, 3-68%] vs. 59.0% [range, 41-70%];P=0.000035). The percentage of CD4+T cells producing IL-2 was negatively correlated with the CsA level(Rc=-0.0821,P=0.00002297) but not with prednisolone or azathioprine doses. Fewer CD8+T cells produced IL-2 in transplant patients compared with controls, but the difference failed to reach statistical significance. The percentage of CD8+T cells capable of producing IL-2 was inversely correlated to CsA levels(Rc=-0.0375,P=0.0011).These data suggest that the functional effects of CsA in transplant recipients can be quantitatively determined and that the capacity of CD4+T cells to produce IL-2 upon stimulation constitutes a functional parameter of CsA effects on the immune system. Prospective studies are required to determine whether this method is useful for clinical monitoring.