BACKGROUND/AIMS: We investigated the antiviral and immunomodulatory effects of a combination treatment using thymus humoral factor-gamma 2 and alpha-interferon in patients with chronic hepatitis B in whom previous monotherapy with interferon had failed. METHODS: Nine HBeAg and HBV-DNA seropositive patients received thymus humoral factor-gamma 2 alone for 2 months, thymus humoral factor-gamma 2 plus alpha-interferon for 2 months and finally alpha-interferon alone for 2 months. RESULTS: Treatment with thymus humoral factor-gamma 2 alone was not associated with any side effects. The interferon-induced lymphopenia was significantly less marked during the combined therapy in comparison to the previous course with interferon alone (mean reduction of lymphocyte counts 33.5 +/- 11.6% versus 56.3 +/- 16.7%, respectively, p < 0.05). The combination of thymus humoral factor-gamma 2 plus interferon showed a significantly more profound inhibition of serum HBV-DNA (mean reduction from the pretreatment level 90.6 +/- 13.3%) compared to the earlier monotherapy with interferon in the same patients (mean reduction 55.5 +/- 34.7%, p < 0.01). As a result of the combined thymus humoral factor-gamma 2 plus alpha-interferon regimen three out of nine patients became HBV-DNA negative and seroconverted to anti-HBe. Thymus humoral factor-gamma 2 appears to exert mainly a functional effect on T lymphocytes, as interleukin-2 production was increased in the majority of treated patients, whilst the expression of lymphocyte activation markers remained unchanged. CONCLUSIONS: These data suggest that thymus humoral factor-gamma 2 may be useful in a combined therapeutic approach in chronic HBV carriers.