Polypyrimidine-tract-binding protein (PTB) is involved in pre-mRNA splicing and internal-ribosomal-entry-site-dependent translation. The biochemical properties of various segments of PTB were analysed in order to understand the molecular basis of the PTB functions. The protein exists in oligomeric as well as monomeric form. The central part of PTB (amino acids 169-293) plays a major role in the oligomerization. PTB contains several RNA-binding motifs. Among them, the C-terminal part of PTB (amino acids 329-530) exhibited the strongest RNA-binding activity. The N-terminal part of PTB is responsible for the enhancement of RNA binding by HeLa cell cytoplasmic factor(s).