The product of the von Hippel-Lindau gene, pVHL, targets the ␣ subunits of the heterodimeric transcription factor hypoxiainducible factor (HIF) for polyubiquitination in the presence of oxygen. The binding of pVHL to HIF is governed by the enzymatic hydroxylation of conserved prolyl residues within peptidic motifs present in the HIF␣ family members. By using a biochemical purification strategy, we have identified a human homolog of Caenorhabditis elegans Egl9 as a HIF prolyl hydroxylase. In addition, we studied the activity of a structurally diverse collection of low molecular weight inhibitors of procollagen prolyl 4-hydroxylase as potential inhibitors of the HIF hydroxylase. A model compound of this series stabilized HIF in a variety of cells, leading to the increased production of its downstream target, vascular endothelial growth factor.