The proto-oncogene c-kit plays an important role in the development and survival of mast cells. Gain-offunction mutations in c-kit are one of the most characteristic events in mast cell leukemia (MCL) but as yet there is no clinically approved treatment for the disease. Here we describe growth inhibition of human MCL cell lines by the use of RNAi against c-kit or its mutant form. Retroviral transduction of HMC1.1 and HMC1.2 cell lines with vectors carrying DNA to be transcribed to RNAi against the wild type or mutant c-kit messengers reduced Kit protein levels considerably, decreased cell proliferation, and increased the apoptotic levels five days after retroviral infection. Thus RNAi targeted against Kit or its mutant form could be considered as a new antiproliferative agent against human mast leukemia cell lines, especially HMC1.2 cells which are resistant to the Kit tyrosine kinase inhibitor, imatinib mesylate.