Lipophilic and hydrophilic extracts from approximately 600 strains of cultured cyanophytes, representing some 300 species, were examined for antiviral activity against three pathogenic viruses. Approximately 10% of the cultures produced substances that caused significant reduction in cytopathic effect normally associated with viral infection. The screening program identified Chroocococcales as commonly producing antiviral agents. Key index words: antiviral, cyanobacteria, cyanophyte, herpes virus, HIV-1, HIV-2, human immunodeficiency virus, natural products, pharmaceutical, respiratory syncytical virus.