A series of 3-aryl-4-arylidene-5-isoxazolones were found to be active as inhibitors of PKC, and in ex-vivo and in vivo models of graft-versus-host disease.The protein kinase C (PKC) family of serine/threonine kinases is central to the regulation of numerous cellular functions as a consequence of its role in signal transduction, 1,2,3 and PKC deregulation or over-activity has been implicated in inflammatory diseases, certain cancers, and other disorders. 4,5 In particular, the role of PKC in the regulation of cell differentiation and proliferation suggests that PKC inhibitors might be useful anticancer agents.6, 7 These potential utilities have prompted widespread screening of synthetic and natural sources for PKC inhibitors, and a number of natural products have been isolated over the past several years which are indeed very potent PKC inhibitors. 8,9,10 Many of these compounds, and their synthetic and semi-synthetic analogues, are in various stages of evaluation as chemotherapeutic agents. The regulation of cell activation and proliferation within the immune system was of interest to us in that inhibitors of PKC also have the potential to be medically useful immunosuppressive agents.11We report here a series of 3-aryl-4-arylidene-5-isoxazolones 3 and 4 which are selective, micromolar inhibitors of PKC, active against a variety of PKC subtypes but selective for PKC over other protein kinases. This pattern of selectivity, and the ease of synthesis, should make these compounds useful as biochemical tools. The compounds are also active in ex vivo and in vivo models of Graft-versus-Host (GvH) disease.