Activation of a leukocyte-type 12-lipoxygenase (12-LO) has been proposed to be an important mechanism for angiotensin II- and glucose-induced vascular smooth muscle cell growth. Currently, no specific pharmacological inhibitors for the leukocyte-type 12-LO are available to test this hypothesis. We have therefore designed a chimeric DNA-RNA hammerhead ribozyme to produce cleavage at the first GUC sequence at nucleotide 7 of porcine leukocyte 12-LO mRNA. The ribozyme was tested in vitro with a 206-base 12-LO mRNA as substrate. We observed that the ribozyme specifically and dose-dependently cleaved porcine leukocyte 12-LO mRNA at the predicted site under physiological temperature. Furthermore, we also efficiently delivered the ribozyme into porcine aortic vascular smooth muscle cells by transfection with cationic liposomes. The ribozyme caused a dose-dependent decrease in levels of porcine leukocyte-type 12-LO mRNA in these cells and was more potent than an antisense oligonucleotide directed against porcine leukocyte 12-LO. The 12-LO ribozyme also attenuated 12-LO protein levels in the cells. The action of the ribozyme was primarily a result of its catalytic activity, since a modified ribozyme that lacks catalytic activity showed reduced effects. This represents the first ribozyme directed against a mammalian LO pathway. These results demonstrate the potential utility of new ribozyme technology to generate novel agents for gene modulation experiments to modify the development or progression of vascular disease in humans.