A water-soluble fraction from the marine diatom Haslea ostrearia was capable to inhibit the in vitro replication of HSV-1 in Vero cells with 50% inhibitory concentration (EC50) of 14 micrograms/ml at a multiplicity of infection of 0.01 ID50/cells. In addition, this fraction delayed the HIV-1-induced syncitia formation on MT4 cells. At concentrations up to 200 micrograms/ml, no cytotoxicity was observed for both the Vero and MT4 cells. The fraction only inhibited the blood coagulation process at concentrations considerably exceeding the EC50.