We examined the ability of dimethylsulfoniopropionate (DMSP), its cleavage products dimethyl sulfide (DMS) and acrylic acid (AA), and the oxidized form of DMS dimethylsulfoxide (DMSO), to inhibit infection of Emiliania huxleyi virus 86 (EhV-86). Infectivity was assessed by plaque assay of viral stock that had been exposed to these compounds. The initial concentrations of the compounds tested were 250 mmol L-1 for DMSP, DMS, and AA, and 14 mmol L-1 for DMSO. These are the maximum concentrations thought to occur in E. huxleyi and therefore the highest EhV-86 might encounter. DMSP and DMSO had no effect on EhV-86; however, both DMS and AA diminished viral titers. Further experiments established that both DMS and AA significantly reduced titers from a concentration of 100 mmol L-1 and that they had a greater antiviral effect when applied in combination. The DMSP system in algae could function as a chemical defense against viral infection that would benefit the surviving cells in the population by reducing infective titers of progeny viruses and therefore decreasing the probability of infection of further cells.