Viral infection of lymphocytes induces the synthesis of interferon (IFN)-alpha and immunoreactive corticotropin (irACTH). We have previously shown the irACTH to be antigenically, structurally, and functionally related (if not identical) to pituitary ACTH. Virus infected lymphocytes also synthesize in endorphin as measured by immunofluorescence. The endorphin-like activity was found to be associated directly with IFN-alpha as well as other, lower molecular weight peptide(s) having no antiviral activity. Crude IFN-alpha at 1000 U/ml exhibited opiate receptor binding activity by inhibiting 47.2% of 3H dihydromorphine (4 X 10(-9) M) binding to mouse brain tissue. Homogeneous HuIFN-alpha (10(8.3) U/mg) also bound to opiate receptors but required 10 times the antiviral activity than crude IFN-alpha for a 50% inhibition of dihydromorphine binding. When injected intracerebrally both crude and homogeneous IFN-alpha induced transient analgesia in mice that was reversible and preventable by the opiate antagonist naloxone. The low molecular weight (less than 10 Kd) ir endorphins purified from the IFN-alpha had no antiviral activity, but may represent the majority of the opiate receptor binding material in the infected lymphocyte culture fluid. These data seem to indicate that the opiate-like side effects of exogenous IFN administration may be due to the IFN-alpha molecule binding to opiate receptors and also may be due to the associated low molecular weight endorphin-like moieties synthesized by lymphocytes.