Aspartic proteinases of vertebrate, fungal and retroviral origin have been characterised by utilisation of chromogenic peptide substrates and synthetic inhibitors as probes. By this means, the molecular topography of sub-sites within the active site cleft of individual enzymes has been elucidated. With suitable selection of residues to occupy each sub-site, the design of effective inhibitors specifically targetted against individual aspartic proteinases has become feasible.