ABSTRACTExtracts of 28 species of marine macroscopic algae collected from various coastal habitats of northern California were examined for antiviral activity against a broad spectrum of mammalian viruses. Ten members of Rhodophyta contained substance(s) which caused greater than a 2 log reduction in the infectivity of herpes simplex virus types 1 and 2. In addition, anti‐Coxsackie Bsvirus activity was detected in extracts ofConstantinea simplexSetchell. The physical and chemical properties of the substance in extracts ofFarlowia mollis(Harvey and Bailey) Farlow and Setchell andC. simplexindicated the active agent was a structural polysaccharid