Prenyltransferase (EC 2.5.1.1) has been obtained from chicken liver in a stable crystalline form. The enzyme has been shown to be homogeneous by polyacrylamide gel electrophoresis at pH 8.4, and by electrophoresis in sodium dodecyl sulfate containing gels. Electrofocusing of the crystalline enzyme results in a single sharp protein peak with a pI of 5.72. The protein is a dimer of molecular weight 86,000 whose subunits were not resolved by gel electrophoresis in sodium dodecyl sulfate. Michaelis constants of 0.5 muM for both isopentenyl pyrophosphate and geranyl pyrophosphate are 3-20-fold lower than those found for prenyltransferase from yeast or pig liver (Eberhardt, N., and Rilling, H. C. (1974), J. Biol. Chem. (in press); Dorsey, J. K., Dorsey, J. A., and Porter, J. W. (1966), J. Biol. Chem. 241, 5353; Holloway, P. W., and Popjak, G. (1967), Biochem. J. 104, 57). The enzyme primarily synthesizes farnesyl pyrophosphosphate from dimethylallyl or geranyl pyrophosphate although some geranylgeranyl pyrophosphate is formed under certain conditons. This is the first preparation of a stable crystalline enzyme of sterol and terpene biosynthesis.