According to the experimental data that IL-11 costimulates the proliferation of human hematopoietic progenitors in vitro the first clinical trials in vivo have been performed. Surprisingly, the patients treated with recombinant Il-11 display decrease in the hematocrit values. This finding is contradictory to previous experimental studies in vitro, where IL-11 was found to be a potent costimulator of eytrhopoiesis. Therefore in order to obtain our own data the influence of IL-11 on human erythropoiesis in vitro has been reevaluated. In performed studies we did not observed any influence of IL-11 on the cloning efficiency of human erythropoietic progenitors in vitro. Our results demonstrate on one side that clinical use of IL-11 in order to stimulate erythropoiesis is not enough theoretically grounded and on the other that the decrease of hematocrit values observed after in vivo treatment with recombinant IL-11 could not be explained simply by the fact of the possibility of the direct inhibition of erythroid colonies formation by Il-11.