Activating mutations of FLT3 have been detected in patients with acute myeloid leukemia (AML). Two distinct types of FLT3 mutations are most common: internal tandem duplication (ITD) of sequences coding for the juxtamembrane domain and point mutations at codon 835 (Asp835) within the kinase domain. Both types of mutations constitutively activate the tyrosine kinase activity of FLT3 in experimental systems and result in factor-independent proliferation of Ba/F3 and 32D cells. Recently, novel mutations within the activation loop were identified in patients with AML: deletion of isoleucine 836 (Ile836del) and an exchange of isoleucine 836 to methionine plus an arginine insertion (Ile836Met+Arg). To examine whether the Ile836 mutations result in constitutive activation of the FLT3 receptor, we introduced both mutant FLT3 cDNAs transiently into HEK 293 cells. Both mutant FLT3 receptors were constitutively autophosphorylated in the absence of ligand and kinase activity led to constitutive activation of downstream signaling cascades as determined by activation of the STAT5 (signal transducer and activator of transcription 5) pathway. When stably expressed in the growth factor–dependent cell lines Ba/F3 and 32D, both deletion and insertion mutants led to factor-independent proliferation, indicating that both mutants have transforming capabilities. We then examined the sensitivity of the FLT3 ITD, FLT3 Asp835Tyr, and the novel FLT3 receptor mutants toward the kinase inhibitors AG1296, PKC412, and SU5614. We show that these FLT3 kinase inhibitors have distinct inhibitory potencies against different activating FLT3 receptor mutants. These results suggest that it may be useful to determine the exact kind of FLT3 mutation when applying receptor kinase inhibitors in clinical trials.