VALIDATE is a hybrid approach to predict the binding affinity of novel ligands for receptors of known three-dimensional structure. This approach calculates physicochemical properties of the ligand and the receptor-ligand complex to estimate the free energy of binding. The enthalpy of binding is calculated by molecular mechanics while properties such as complementary hydrophobic surface area are used to estimate the entropy of binding through heuristics. A diverse training set of 51 crystalline complexes was assembled, and their relevant physicochemical properties were computed. These properties were analyzed by partial least squares (PLS) statistics, or neural network analysis (SONNIC), to generate models for the general prediction of the affinity of ligands with receptors of known three-dimensional structure. The ability of the model to predict the affinity of novel complexes not included in the training set was demonstrated with three independent test sets : 14 complexes of known three-dimensional structure including 3 DNA complexes, a class of compound not included in the training set, 13 HIV protease inhibitors fit to HIV-1 protease, and 11 thermolysin inhibitors fit to thermolysin.