Poliovirus proteins were labelled in vivo with [35S]-methionine, and the major products of translation and cleavage were separated by electrophoresis and compared in terms of two-dimensional tryptic peptide maps visualized by autoradiography. The main intermediates p110 and p90 had few or no methionine-labelled sequences in common, but were both contained in, and therefore almost fully account for, the presumed primary translation product p210. The sequences of p79, a major stable product of cleavage and a non-structural protein, were almost completely contained in p90, which in turn is the major component of the larger intermediates p168 and p155. P110 is confirmed as the precursor of virus particle protein, and VP0 as the precursor of VP2. However, the sequences of p31, the other major product of translation that is not a stuctural protein, were not contained in any of the viral polypeptides mentioned above.