Interleukin-3 is a multipotential hematopoietic growth factor, which like other colony stimulating factors (CSFs) is effective "in vitro" stimulation of the mature cells function. It was found that IL-3 synergistically with GM-CSF and G-CSF stimulated the proliferation of the granulocytes. Therefore the purpose of this investigation was the evaluation "in vivo" of the influence of IL-3 on the phagocytosis, bactericidal activity, and enzyme activities of granulocytes. IL-3 was injected into mice subcutaneously during 5 days in dose 1 microgram/kg/d. The examination of the percent of cells phagocytizing bacteria (Staphylococcus aureus), NBT test and bactericidal activity, were performed every day and evident increase of the tested parameters was found. Additionally the enzyme activities in primary granules were measured and showed on increase of acid phosphatase and peroxidase activity.