Proliferation and functional activation of endothelial cells within a tissue site of inflammation are regulated by humoral factors released by cells, such as T lymphocytes and monocytes, infiltrating the perivascular space. In the present study we investigated the effects of interleukin 3 (IL-3), an activated T lymphocyte-derived cytokine, on cultured human umbilical vein endothelial cells (HUVEC). Proliferative activity, evaluated both by estimation of the fraction of cells in the S phase and by direct cell count demonstrated that IL-3, at the dose of 25 ng/ml, enhances more than threefold both DNA synthesis and cell proliferation above baseline control conditions. Binding studies with radioiodinated ligand demonstrated that HUVEC constitutively express a smaller number of IL-3 binding sites (approximately 99 binding sites per cell, with an apparent Kd of 149 pM). Accordingly, molecular analysis showed the presence of transcripts for both alpha and beta subunits of the IL-3 receptor. Functional activation of endothelial cells was evaluated by the expression of the endothelial-leukocyte adhesion molecule 1 (ELAM-1) transcript and by leukocyte adhesion. The ELAM-1 gene transcript was clearly detectable 4 h after IL-3 addition and started to decrease after 12 h. Moreover, IL-3-induced ELAM-1 transcription was followed by enhanced adhesion of neutrophils and CD4+ T cells to HUVEC. The findings that IL-3 can stimulate both proliferation and functional activation of endothelial cells suggest that this cytokine can be involved in sustaining the process of chronic inflammation.