Ethanolic extract of the green part of Chamaecyparis Lawsoniana was tested for antiviral activity and toxicity in tissue culture. All experiments were carried out in confluent human embryo lung fibroblasts. Treatment of fibroblast cells with extracts after viral inoculation was effective in inhibiting the replication of herpes simplex virus type 2 (HSV-2). The critical time for virus inhibition was 4 to 5 h after virus absorption. The antiviral activity was assayed employing the techniques of viral plaque and yield reduction. Toxicity of Chamaecyparis Lawsoniana in uninfected cells was studied as alteration of cell morphology, cellular viability and inhibition of host cell DNA synthesis. Herpes simplex virus inhibition occurred in presence of extract concentration of 0.5 micrograms/ml, whereas concentrations exceeding 5 micrograms/ml and 140 micrograms/ml were found to be cytotoxic when evaluated with inhibition of host DNA synthesis and cellular viability respectively. Results suggest that further investigations concerning the isolation of substances responsible for antiviral activity and an effort to define the mechanisms of action are warranted.