The effects of Chinese medicinal herbal drugs (CMH; Daphne genkwa, Wikstroemia indica, Croton oil) were studied for Epstein-Barr virus (EBV) early antigen (EA) induction in established nasopharyngeal hybrid cells. Both ether and water extracts of CMH were found to have inducing activity. However water extracts of the same herbs were not as strong as that of other extracts. The EA positive-cell rate was from 18.2 to 42.2% in ether extracts and 1.0 to 3.8% in water extracts at 10 microgram/ml of the concentration. N-Butyrate alone showed a 40.2% positive rate and in the both treatment of water extracts, a combination effects was seen in induction of the EBV-EA. This in vitro system for the induction of EBV-EA was thought to be useful to determine what is the causal factors for activation of EBV in vivo.