The hyperpolarizabilities of 2-amino-5-nitropyridine-L-(+)-tartrate (ANPT) clusters that represent parts of the ANPT crystal have been calculated by a finite-field method using the semi-empirical AM1 Hamiltonian. Calculated results for several kinds of clusters have indicated that the hyperpolarizability in the crystal was strongly enhanced by two kinds of intermolecular interactions. One was the pyridinium-pyridinium interaction, and the other was the pyridinium tartrate interaction. The former was dominantly contributed by the interaction among the pyridinium ions lying along their amino-to-nitro axis. The latter was the interaction between the hydrogen-bonded ions, and was suggested to include the intermolecular charge transfer. The enhanced hyperpolarizability was approximately three times as large as that without the interactions.