Incubation of the alphaherpesviruses pseudorabiesvirus (PRV) and bovine herpesvirus 1 during infection of cell cultures with an extract prepared from the leaves of Acanthospermum hispidum impaired productive replication of these viruses in a concentration-dependent manner whereas propagation of classical swine fever virus, foot-and-mouth disease virus and vaccinia virus was not affected. The 50% inhibitory concentration for cell growth (IC50) was 107 +/- 5 mu l/ml, and the concentration reducing PRV yield by 1 log(10) (90% effective concentration, EC90) was 8 +/- 3 mu l/ml. The selectivity index calculated as the IC50/EC90 ration was 13 +/- 4. Delineation of the mechanism of the antiviral activity demonstrated inhibition of alphaherpesvirus attachment to and, to a lesser extent, penetration into the cells. In contrast, viral gene expression was not inhibited by the extract when added after entry of virions into the target cells. Reduced antiviral activity of A.h. against PRV deletion mutants lacking glycoprotein C (gC) or glycoproteins gC, gE, gG and gI altogether indicated that gC alone and/or viral attachment complexes of which gC is a component constitute the target structures for A. hispidtum. (C) 1997 Elsevier Science B.V.