The platelet type 12 lipoxygenase (12-LOX) adds molecular oxygen to C-12 arachidonic acid to yield 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid. It has been suggested that 12-LOX and its metabolites play an important role in tumor angiogenesis. A hammerhead ribozyme (Rz) targeted to the first GUC site within the 12-LOX mRNA was designed and cloned into an in vitro transcriptional or mammalian expression vector. In vitro, the Rz was able to cleave its substrate efficiently in a time-dependent manner. Under multiple turnover conditions, the Rz performed well at 37 degrees C, with a further improvement at 50 degrees C. When cloned into a mammalian expression vector, pSV2neo, the Rz construct efficiently decreased the level of 12-LOX mRNA in stably transfected human erythroleukemia cells to levels that were undetectable by Northern blot analyses. 12-LOX enzyme activity assays showed that Rz significantly reduced the 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid production in human erythroleukemia cells; this effect was sustained for up to 6 months in cell culture. The Rz developed in this study may represent a powerful tool for potential applications, ranging from an understanding of tumor angiogenesis to cancer gene therapy.