Abstract Hydrolyzed potato starch, at concentrations of 10 and 11%, has been successfully incorporated into a nutrient overlay and used for the plaque assay of Vaccinia and Herpes simplex viruses. It was well tolerated by cells of different origin. After adsorption periods of up to 2 hours, twice as many Vaccinia plaques appeared under starch as compared to agar. With Herpes simplex virus, after an adsorption period of 30 minutes, 5 times as many plaques were obtained under starch as compared to agar, and the plaques under starch were furthermore larger. Suggestive evidence was obtained that agar blocks the penetration of some cell-attached virus, whereas starch does not.