In vitro culture of hairy roots of Phyllanthus amarus induced by Agrobacterium rhizogenes was established. Their growth and ability for in vitro inactivation of hepatitis B virus surface antigen was studied and compared with adventitious roots grown in vitro. The selected hairy root clone HR-1 was capable of growing at a very fast rate, and an approximately 900-fold increase in weight of root biomass was achieved after 4 wk of culture in hormone-free quarter-strength liquid Murashige and Skoog medium with continuous agitation. Non-transformed roots cultured in the presence of 1.0 mg l⁻¹ (5.71 μM) indole-3-acetic acid increased by 330-fold. The immuno-inactive property of roots was maximal in the crude extract. The hairy roots were shown to possess 85% inhibition (in contrast to 15% in the control) in binding of hepatitis B surface antigen (HBsAg) to its antibody (anti-HBs) after 24 h of incubation with HbsAg-positive sera in vitro at 37°C. Out of three fractions selected on the basis of molecular weight components of the extract, the Fraction III containing comparatively lower molecular weight substances (≤3500) yielded the highest activity. The extract from non-transformed roots was found to possess similar efficiency (87% inhibition). The levels of activity in both types of in vitro-raised roots were higher than those of naturally occurring roots and leafy shoots. The ability of P. amarus hairy root cultures to yield high biomass with the anti-viral property at high levels may provide an alternative source of raw material for more detailed study in the field of pharmaceutical research.