The invention relates to antibodies directed against human Leukemia Inhibitory Factor (LIF) and to a hybridoma cell line producing said antibodies. The invention also relates to a method for blocking/inhibiting the proliferation of stem cells, and to an in vitro method for the diagnosis of diseases associated with unwanted cell proliferation in a subject or for determining the predisposition of a subject to suffer from said disease associated with unwanted cell proliferation, or for prognosis of average life expectancy of a subject suffering from said disease. The therapeutic potential of said antibodies is based on observing that the inhibition of LIF can be used in therapeutic compositions for the treatment of diseases associated with unwanted proliferation.