Several antitumor substances extracted from cones of various pine trees inhibited the plaque formation of Herpes simplex virus types 1 and 2 (HSV-1, HSV-2) strains in African green monkey kidney cells and human adenocarcinoma cells. The 50% effective dose of the most active fraction, Fr. VI (0.3 micrograms/ml) was 0.001 times its 50% cytotoxic dose (greater than 300 micrograms/ml). The anti-HSV activity of various pine cone extracts increased with their acidity. Identification of the polyphenol groups as donors of the acidity was further supported by the observation that the anti-HSV activity of the natural polyphenolic products, such as tannin and lignin, significantly exceeded that of other natural or chemically modified antitumor polysaccharides. An experiment using radiolabeled virus particles indicated that the anti-HSV effect of both Fr. VI and lignin was attributable to interference with virus adsorption to these cells rather than to inhibition of virus penetration into the cells.