Ab initio Hartree-Fock SCF calculations with the molecular fragment technique have been performed on several drugs which exhibit activity in the rat passive cutaneous anaphylaxis (PCA) assay. Representative molecules of the following types were included in the series: oxanilic acids, 1,4-dihydro-4-oxoquinaldic acids, and 4-oxo-4H-1-benzopyran-2-carboxylic acids. A quantitative relationship has been established between the observed biological activity and an electronic index obtained from the calculations. The correlation is rationalized in terms of charge-transfer stabilization of the drug-receptor complex.