The pattern of viral polypeptide synthesis and cleavage in poliovirus-infected cells was shown by autoradiography to be considerably more complex than previously thought. In normal growth, at least 26 distinct polypeptides were found, and various modifications of the cleavage process revealed a total of at least 34. Most of the new polypeptides were minor components that were unstable during a chase. Different cultural modifications led to different polypeptide ratios, and it appeared likely that several cleavage activities were involved . Minor differences were found in the polypeptide contents of cytoplasmic extracts and whole infected cells. The complexity of the cleavage pattern necessitated a new nomenclature based on mol. wt. (e.g. Pp110, "poliovirus protein" of 110000). Particular attention was paid to mol. wt. determinations, notably in the use of internal protein standards and more fully denaturing gel conditions. The size of the "primary translation product" of poliovirus RNA was found to be 210000 daltons, so that either 20% of the viral genome is not translated in vivo, or some is read as a smaller independent translation unit.