Poliovirus induces a shut-off of messenger RNA (mRNA) translation by the proteolysis of a 220 kDa protein from the eukaryotic initiation factor eIF-4F, and by the phosphorylation of eIF-2. The absence of eIF-4F inhibits the initiation of translation dependent on cap structure recognition. Poliovirus RNA lacks cap structure and translates by a cap-independent mechanism which requires internal ribosomal entry. The poliovirus 5' untranslated region (5'UTR) contains the structural elements for cap-independent translation called internal ribosomal entry site (IRES element). Several cellular proteins have been described interacting with different segments from poliovirus 5'UTR. We have studied the specific interaction between 57/60, 52, and 35 kDa cellular proteins with poliovirus nt 275 to 636 and full length 5'UTR. By Western blot assay the 57/60 protein was identified as a pyrimidine tract binding protein (PTB), and the 52 kDa protein as a La-autoantigen. La and PTB are nuclear proteins involved in RNA polymerase III transcription termination and splicing, respectively.