Rational ligand design is a complex problem that can be divided into three parts: the search for optimal positions and orientations of functional groups in the binding site, the connection of such positions to form candidate ligands, and the estimation of their binding constants. Approaches for addressing the first two parts of the problem are described in the present work. They are applied to the construction of peptide ligands in the binding site of the human immunodeficiency virus 1 (HIV-1) proteinase. The primary objective is to test the method by comparison of the results with the MVT-101 complex structure for which coordinates are available; the results obtained with the liganded and unliganded proteinase structure are used to examine the utility of the latter for binding studies. A secondary objective is to show how to find new inhibitor candidates. The multiple copy simultaneous search (MCSS) method is utilized to search for optimal positions and orientations of a set of functional groups. For peptide ligands, functional groups corresponding to the protein main chain (N-methylacetamide) and to protein side chains (e.g., methanol, ethyl guanidinium) are used. The resulting N-methylacetamide minima are connected to form hexapeptide main chains with a simple pseudoenergy function that permits a complete search of all possible ways of connecting the minima. Side chains are added to the main-chain candidates by application of the same pseudoenergy function to the appropriate functional group minima. A set of 15 hexapeptides with the sequence of MVT-101 is then minimized by a Monte Carlo scheme, which allows for escape from local minima. Comparison of the MCSS results with the structure of MVT-101 in the HIV-1 binding site showed that all of its functional group positions correspond (within 2.4Å) to some (usually more than one) MCSS minima. There were also many other low-energy MCSS minima which do not appear in any known inhibitors, e.g., methyl ammonium minima in the neighborhood of the catalytic aspartates. Among the 15 lowest minima are seven hexapeptides with the same main-chain orientation as the one found by X-ray crystallography for the inhibitor MVT-101 in the binding site and eight with the main chain oriented in the opposite direction; the latter tend to be more stable. [Addendum: These results are in agreement with recent high-resolution crystallographic data provided after the study was completed. They show that the MVT-101 binds in two orientations and that the published orientation represents the minor conformer (M. Miller et al., private communication). A set of terminal blocked dipeptides were constructed from low-energy MCSS minima at one open end of the HIV-1 aspartic proteinase binding site and their interactions with the protein were analyzed. It was shown that some of the dipeptides can be connected to known hexapeptide ligands. The paper demonstrates that the combination of a method for an exhaustive search of the binding site for functional group minima (MCSS) with a highly efficient method for constructing molecules from them provides a novel and effective approach to the theoretical design and docking of candidate peptide ligands. The results of the present analysis suggest several modifications of MVT-101 that may have increased affinity and/or specificity for the HIV-1 aspartic proteinase binding site