Data are presented which indicate that seawater possesses antiviral properties of a biological nature associated with specific marine bacteria. Twelve samples of natural seawater from points along the Mediterranean and Red Sea coasts of Israel show the ability to reduce the concentration of poliovirus inoculums about 99.99% in 7 days at 20°C. A number of pure culture isolates of marine bacteria were shown to possess this same activity. Seawater samples treated to remove viable marine bacteria lose their antiviral properties. Field and laboratory studies show that poliovirus has a T90 of 1 to 2 days and are considerably more resistant to the natural inactivation properties of seawater than coliforms. The results of this study may cast some doubts on the usefulness of coliform standards and dieaway rates in designing ocean outfalls.