Another procedure for concentrating enteric viruses from dilute suspension-the aqueous polymer two phase system-was also being studied seriously during the 1960s (Philipson et al. 1960) . Two groups reported their studies of this method at the 1965 symposium (Lund and Hedstrom 1967; Shuval et al. 1967 )), and one reported further study in a journal (Lund and Hedstrom 1966) . The method entailed adding polyethylene glycol 6000 and sodium dextran sulfate to the sample; the thoroughly mixed suspension was placed in a cold room to separate for 18-24 h, and the small, lower phase (formed by the sodium dextran sulfate) was harvested and tested for virus. We tested the procedure with seven types of enteroviruses plus influenza A virus and found that the sodium dextran sulfate was apparently inhibitory to coxsackievirus B2, echovirus 6 (EC6), and influenza A virus . The last of these was probably of no consequence, but the inhibitory action of the sodium dextran sulfate seemed likely severely to bias surveys of water or wastewater such as had been reported previously (Lund and Hedstrom 1967) . We later showed that substituting dextran T-500 for the sodium dextran sulfate mitigated the inhibition problem (Grindrod and Cliver 1970) ; however, the method was relatively ponderous and slow; so it found limited use. It eventually developed that viruses could be precipitated with polyethylene glycol, without dextran for phase separation (Lewis and Metcalf 1988 ).