The enhancement by diethylaminoethyl-dextran (DEAE-D) of the infectivity of poliovirus ribonucleic acid (RNA) for cell cultures was demonstrated by infectivecenter as well as by plaque assays, both in nonprimate (L) and primate cell systems (MK, HeLa, LLC-MK2). The sensitivity of plaque assays was greatly improved by using a tris(hydroxymethyl)aminomethane-buffered synthetic medium (basal medium Eagle) and freshly confluent cell monolayers. Enhancement of nucleic acid infectivity was directly

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... pendent on the molecular weight of the DEAE-D. Two observations bearing on the action of DEAE-D appeared important: ribonuclease activity was reduced by DEAE-D, and cells pretreated with DEAE-D remained susceptible to infection with RNA in isotonic medium. Appreciable susceptibility of the treated cells persisted for at least 2 hr; the susceptible state could be reversed at will by an application of heparin. Enhancement of nucleic acid infectivity was independent of an effect of DEAE-D on intact virus and agar inhibitors.