After receptor binding, growth hormone (GH) induces GH receptors (GHR) dimerization and JAK2 is activated after its association with a dimerized GHR, stimulating the tyrosyl phosphorylation of insulin receptor substrate-1 (IRS-1), IRS-2 and Shc proteins. G120K-PEG, a GH antagonist is produced by a mutation that blocks GH action by preventing the GHR dimerization. This study shows that the inhibitory effect of G120K-PEG was maximal with a GH:G120K-PEG ratio of 1:100, as no increase in JAK2 tyrosyl phosphorylation was observed with this dose of GH. When the dose of GH was increased and with a GH:G120K-PEG ratio of 1:10 some tyrosyl phosphorylation of JAK2 could be observed. Additionally, GH-induced IRS-1, IRS-2 and SHC tyrosyl phosphorylation was inhibited approximately 50% at equimolar concentrations of the antagonist of GH and almost abolished with a GH:G120K-PEG ratio of 1:100. The results clearly show that G120K-PEG inhibits GH signal transduction in mouse liver.19265-74