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Poliovirus type 1 enters HeLa cells by receptor-mediated endocytosis as an intact virus. Up to 30 min after adsorption complete virus particles still containing VP4 and sedimenting with 156 S could be recovered from the cells. These virus particles were N-antigenic and infectious. Thirty minutes after adsorption the recovery of intact and infectious virus decreased. This decrease presumably reflects viral uncoating in the acidic endosomes and/or lysosomes because virus particles could be localized in endosomes at this time. The direct involvement of clathrin-coated structures in the endocytosis of poliovirus has been deduced from the enclosure of poliovirus in coated vesicles at 10 min after adsorption. At this time intact and infectious virus could be recovered only after the coated vesicles were disrupted by treatment with 0.5 M Tris at pH 7.0.