Peripheral blood mononuclear cells (PBMC) cytotoxic activity, percentage of natural killer (NK) cells and T lymphocyte subpopulations in PBMC of 24 patients suffering from chronic active hepatitis B (CAH-B) have been studied. In comparison with healthy subjects CAH-B patients had significantly enhanced both lymphocyte T suppressor (Ts) and NK cell number. Thirteen CAH-B patients with higher Ts number and low lymphocyte T helper/T suppressor ratio (Th/Ts) were subjected to immunostimulation by thymosine factor x (TFx) treatment. After 1 year of TFx treatment all the CAH-B patients improved clinically, the Th/Ts ratio was brought back to normal and 80 per cent of HBe Ag carriers seroconverted to anti-HBe. After one year of TFx administration no significant changes were observed in PBMC cytotoxicity and NK cell number. TFx appears to be a very effective therapeutic agent in some forms of CAH-B. The obtained results show that TFx normalises biochemical parameters of liver lesion and Th/Ts ratio without any effect on NK cell activity in CAH-B patients.