Hammerhead ribozymes containing 2'-fluoro- or 2'-aminonucleotides were prepared by automated chemical synthesis. Incorporation of 2'-fluorouridines, 2'-fluorocytidines or 2'-aminouridines did not appreciably decrease catalytic activity. The presence of 2'-aminocytidines, however, reduced the activity about 20-fold. No catalytic activity could be measured for ribozymes in which all adenosines were replaced by the 2'-fluoro analogue in presence of MgCl2. No single position could be found responsible for this loss of activity. In an attempt to construct ribozymes to hydrolyse HIV-RNA in the 5'-LTR region several constructs were tested on synthetic substrate as well as on run-off transcripts of about 1000 nucleotides length.