The study by the molecular orbital theory displayed that morphine and lysine make two types of the interactions between them: type (A) by three hydrogen bondings and type (B) by one hydrogen bonding accompanied with a proton transfer. The stabilization energies were 45.3 and 34.9 kcal/mol for type (A) and type (B), respectively. The characters of these interactions are striking compared to the interactions of morphine with the other amino acids, suggesting that lysine is the binding point of morphine in the mu-opioid receptor.