Mannan sulfate, a novel sulfated polysaccharide, was prepared and investigated for its activity against human immunodeficiency virus type 1 (HIV-1) in vitro. Mannan sulfate completely inhibited HIV-1-induced cell destruction and viral antigen expression in HIV-1-infected Molt-4 (clone 8) cells at a concentration of 4 micrograms/ml. The 50% antiviral effective doses obtained with mannan sulfate in Molt-4 (clone 8) cells and in MT-4 cells were 1.5 and 9.3 micrograms/ml, respectively. No toxicity for Molt-4 (clone 8) cells or MT-4 cells was observed at a concentration of 4,000 and 2,500 micrograms/ml, respectively. Mannan sulfate was also inhibitory to other enveloped viruses, i.e. herpes simplex virus types 1 and 2, vaccinia virus and vesicular stomatitis virus. These results suggest that mannan sulfate may be useful for the chemotherapy of various viral infections, including those causing and associated with AIDS.