FK-506 is known to suppress the transcription of several genes encoding cytokines (e.g., IL-2, IFN-γ) thought to play an important role in the allograft response. This general inhibition of cytokine gene transcription and protein production was previously thought to be the main mechanism by which FK-506 suppressed the immune response. We have studied the pattern of cytokine suppression caused by FK-506 in differentiated murine Th 2 cell lines. Supernatants from Th2 cells treated with FK-506 showed marked suppression of IL-4 but only moderate suppression of IL-10 levels. To determine whether this differential effect on IL-4 and IL-10 could also be seen at the mRNA level, total cellular RNA was isolated from cells treated with (1) media, (2) Con A (2 μg/ml), (3) FK-506 (50 ng/ml), or (4) Con A + FK-506. Reverse transcription-polymerase chain reaction and northern blot analysis were used to measure IL-4 and IL-10 mRNA. Similar to results at the protein level, FK-506 suppressed steady state levels of IL-4 mRNA markedly but had a lesser effect on steady state levels of IL-10 mRNA. Furthermore, FK-506 completely abrogated Con A-induced upregulation of IL-4 mRNA, but only slightly suppressed Con A-induced upregulation of IL-10 mRNA. IL-10 (cytokine synthesis inhibitory factor) is a cytokine with immunosuppressive properties that itself inhibits the production of IL-2 and IFN-γ by murine helper cells. IL-10 also downregulates MHC II expression and antigen presentation by monocytes. Therefore, the ability of FK-506 to differentially suppress the mRNA levels of immunostimulatory cytokines such as IL-2, IL-4, and IFN-γ more than the mRNA levels of the immunosuppressive cytokine IL-10 suggests that the effective immunosuppression seen in vivo with FK-506 may be a combination of these 2 effects.