The hammerhead ribozyme belongs to the class of molecules known as antisense RNAs. However, because of short extra sequences that form the so-called catalytic loop, it can act as an enzyme. Since the catalytic domain captures magnesium ions and magnesium ions can cleave phosphodiester bonds, hammerhead ribozymes are recognized as metalloenzymes. In general, the cleavage of phosphodiester bonds involves acid/base catalysis, with proton transfer occurring in the transition state. When the possibility of such a proton-transfer process was examined by measuring solvent isotope effects, it became apparent that no proton transfer occurs in the transition state during reactions catalyzed by a hammerhead ribozyme. It is likely, therefore, that hammerhead ribozymes exploit the general double-metal-ion mechanism of catalysis, with Mg2+ ions coordinating directly with the attacking and leaving oxygen moieties. Since the hammerhead ribozyme is one of the smallest RNA enzymes known and has potential as an antiviral agent, thus ribozyme has been extensively investigated for applications in vivo. Ribozymes are described that have possible utility as agents against HIV-1.