In an effort to define immunobiological parameters identifying "responders" vs "non-responders" to IFN among hepatitis patients, 16 patients with chronic active hepatitis were screened for changes of Natural Killer cell activity (NK). 10/16 patients replicated the hepatitis B virus (HBV-DNA positive) whereas 6/16 replicated the defective B virus associated delta virus (HDV-RNA positive). Patients received 9 MU/3x/weekly/3 months of recombinant IFN alpha A. Mean NK activity of the HBV-DNA patients rose significantly from 29.9 +/- 5.3 to 45 +/- 4.7 during therapy, whereas the 6/16 HDV-RNA positive patients did not show any significant increase of NK activity. Interestingly, individual HDV-RNA positive patients exhibiting boosted NK activity also showed improvement of disease confirmed by clearance of intrahepatic delta antigen at one year. No such a correlation was found amongst the HBV-DNA positive patients. These data indicate that in spite of widespread individual variability, IFN-mediated NK boost may herald delta clearance and help in identifying "responders" and "non-responders" in IFN trials.