Synthetic interferons (IFNs) are used in the treatment of several types of cancer. In addition to an antitumor effect, IFNs show antiangiogenic activity. The aim of this study was to investigate the effects of IFN-α and IFN-β on human micro- and macrovascular endothelial cells in vitro [human micro vascular lung endothelial cells (HMVEC-L) and human umbilical cord endothelial cells (HUVEC)]. By immunohistochemical staining and quantitative reverse transcriptase (RT)-polymerase chain reaction, we studied expression of type I IFN receptors. We evaluated the effects of IFN-α and IFN-β on the proliferation (DNA content), apoptosis (DNA fragmentation by enzyme-linked immunosorbent assay), and cell cycle distribution (flow-cytometric analysis) of endothelial cells. HUVEC and HMVEC-L cells show comparable expression level of the distinct IFN receptor subtypes. Proliferation of HMVEC-L and HUVEC was inhibited by IFN-β (the half maximal inhibitory concentration [IC(50)] = 60 and 90 IU/mL, respectively), but not by IFN-α at a dose up to 1,000 IU/mL. An interesting and unexpected observation was an inhibition of apoptosis by IFN-β. After 72 h of treatment with IFN-β. Cell cycle inhibition occurs in late S-phase in both cell lines. In conclusion, only IFN-β, not IFN-α (10-1,000 IU/mL), has an inhibitory activity on endothelial cell proliferation. Surprisingly, apoptosis was decreased by IFN treatment, whereas inhibition of proliferation is caused by cell cycle arrest in late S-phase.