Peptide-chain initiation with LSc poliovirus was more resistant to hypertonic medium than peptide-chain initiation with Mahoney poliovirus. Protein synthesis of LSc virus retained its relative resistance to high osmolarity created by the addition of excess NaCl to the medium in cells coinfected with Mahoney virus. The data indicate that peptide-chain initiation with LSc virus is intrinsically more resistant to high osmolarity than that of Mahoney virus rather than reflecting different permeability changes in cells after infection. Two Mahoney virus mutants harboring deletions at the 5' end of the viral chromosome exhibited the same sensitivity to excess NaCl as parent virus, suggesting that the original chromosomal region for peptide-chain initiation has not been severely altered by the deletions.