[0047] This invention discloses a process of production of antiviral polysaccharides derived from extracellular culture filtrate of thraustaochytrids, belonging to the marine protistan group of Labyrinthulomycetes. The antiviral polysaccharide shows a broad-spectrum antiviral activity, being active against viruses such as the enterovirus, retrovirus, adenovirus and cytomegalovirus.