The opiate abstinence syndrome represents a fundamental feature of the addictive process. The present study demonstrated that: 1) recombinant leukocyte A Interferon (alpha-IFN) injection prior to chronic morphine treatment reduces addiction liability; and 2) alpha-IFN and not human gamma-IFN injection to morphine dependent rats significantly modifies the naloxone-induced abstinence syndrome in a characteristic dose response manner. Two hypotheses concerning IFN's action were discussed.