An extract of the cactus plant Opuntia streptacantha inhibited intracellular virus replication and inactivated extracellular virus. Inhibition of virus replication also occurred following pre-infection treatment--a favourable finding in terms of in-vivo limitation of virus disease. There was inhibition of both DNA and RNA virus replication, for example, herpes simplex virus, equine herpes virus, pseudorabies virus, influenza virus, respiratory syncytial virus and human immunodeficiency virus, with normal protein synthesis in uninfected cells at extract concentrations which were 15-fold in excess of 50% viral inhibitory concentrations (1 mg/ml). The active inhibitory component(s) of the extract appeared to be protein in nature and resided mainly in the wall of the plant rather than in the cuticle or inner sap. The extract was non-toxic on oral administration to mice, horses and human patients; the non-toxicity of intravenous administration of 70 mg to a mouse representing at least fifty tissue culture 50% viral inhibitory dosages encourages clinical trial of this extract in virus disease of human and veterinary species.