At the request of the Calgon Company of Pittsburgh, we tested a product called Cat-Floc (a polydimethyldiallyl ammonium chloride, MW *500,000) for virus removal when used as a primary coagulant or coagulant aid in treatment of drinking water and wastewater (Cliver 1971) . Results were encouraging, and we were left with a great deal of the product when the trials ended. Oysters (Crassostrea virginica and Ostrea edulis) were experimentally inoculated with enteroviruses and minced with scissors, then stirred with 100 ml PBS to which was added 2 ml of a 1% Cat-Floc solution; after 5 min of stirring and 15 min of settling, the suspension was filtered by pressing in a potato ricer (Kostenbader and Cliver 1972) . The extract could be concentrated by ultracentrifugation or ultrafiltration. Quantitative virus recoveries exceeded 80%, but it was shown that poliovirus neutralized with coproantibody was not reactivated by this process. CA9, coxsackievirus B2, and EC6, which had been inhibited in previous studies with polymers Cliver 1969, 1970) , were not affected by the Cat-Floc. A number of other groups have since used Cat-Floc in extracting viruses from mollusks, crustacea, and estuarine sediments (Johnson et al. 1981; Landry et al. 1982; Richards et al. 1982; Seidel et al. 1983; Wait and Sobsey 1983) .