HIV-1 protease is a homodimeric enzyme. A beta-sheet consisting of the four terminal segments provides the main driving force for dimerization of the per se inactive protomers. Several short peptides with sequences related to the terminal sequences of the protease are able to inhibit dimerization by blocking the 'interface' part of the monomers. From the structures of such inhibitory peptides a 'pharmacophore' could be derived. By using a prominent distance from this pharmacophore scaffold for a library search (Cambridge Structural Database), non-peptide inhibitors of HIV-1 protease with polycyclic triterpene structure could be found. The IC50 constants of these compounds are near 1 microM. One of the triterpenes, the ursolic acid (Ki = 3.4 microM), was further kinetically analysed (according to Zhang). The shape of the graph confirms the expected mechanism of dimerization inhibition.