The binding constants and structural components of 200 drugs and enzyme inhibitors have been used to calculate the average binding energies of 10 common functional groups. As expected, charged groups bind more strongly than polar groups, which in turn bind more tightly than nonpolar groups. The derived intrinsic binding energies (in kcal/mol) are (i) charged groups, CO-2, 8.2; PO2-4, 10.0; N+, 11.5; (ii) polar groups, N, 1.2; OH, 2.5; CO, 3.4; O or S ethers, 1.1; halogens, 1.3; (iii) nonpolar groups, C (sp2), 0.7; C (sp3), 0.8. These values may be used to determine the goodness of fit of a drug to its receptor. This is done by comparing the observed binding constant to the average binding energy calculated by summing the intrinsic binding energies of the component groups and then subtracting two entropy related terms (14 kcal/mol for the loss of overall rotational and translational entropy and 0.7 kcal/mol for each degree of conformational freedom). Drugs that match their receptors exceptionally well have a measured binding energy that substantially exceeds this calculated average value--examples include diazepam and biotin. Conversely, if the observed binding energy is very much less than the calculated average value, then the drug apparently matches its receptor less well than average. Examples of this type include methotrexate and buprenorphine.